I was browsing through the dictionary the other day with a friend, as people do, and we learned about something that bears and humans have in common. I had just bought a 1934 edition of The Concise Oxford Dictionary of Current English, a lovely little book with a soft leather cover, gilt-edged leaves, and a ribbon for marking your place. It cost only $1 at a used book sale. It looked to me like the type of thing a poet might slip into her backpack, right next to the speckled black and white composition book.

My friend turned to the entry for one of our favorite animals, the bear, which, we learned, is a plantigrade quadruped. Plantigrade? Flip, flip, past chorography and gurry and odium. Turns out that bears and humans are both plantigrade; that is, we place the entire sole of the foot on the ground when walking. This is in contrast to digitigrade animals (flip, flip, back past lepidopterous and etui), which walk on their toes (or digits) without placing the heel to the ground (cats and dogs, for example).

It wasn’t until later that I wondered if plantigrade is related to plantar, as in plantar wart. Indeed it is; the two are connected by their shared descent from the Latin word planta, or sole of the foot, which is where plantar warts appear. Planta also means sprout or cutting, and in fact the noun plant, as in those green and gold things I see outside my window, may even come from the Latin verb plantare, which means to push into the ground with the feet. From what I know of field botanists, who do a lot of hiking, the connection between plants and feet on the ground still seems appropriate.

The –grade part of plantigrade, by the way, comes from the Latin word gradus, or step, and appears in other words to describe the way something moves. A moon or planet with a retrograde orbit or spin, for example, moves clockwise (east to west), opposite to the direction of most astronomical bodies. If you know what an ungulate animal is, you can probably figure out what unguligrade means. And now, if you will excuse me, it’s time for me to make my plantigrade way back to the dictionary for more browsing.

If you love science, history, and words, it’s not every day that you find a book that addresses all of those interests, and it’s even rarer to find one that’s also engagingly written and great fun to read. I was lucky enough to have just such a book recommended to me recently: The Mountains of Saint Francis: Discovering the Geologic Events That Shaped Our Earth by Walter Alvarez (find in library). And now I’ll pay it forward by recommending the book to you.

You may be familiar with Alvarez (a geologist at the University of California, Berkeley) because of his key role in the discovery of the impact that ended the Cretaceous period and caused a mass extinction, which he wrote about in T. rex and the Crater of Doom  (find in library). He has become interested in Big History, a new discipline that studies and presents history as an integrated whole, from the Big Bang to now. This interest is evident in The Mountains of St. Francis, which I enjoyed greatly. It’s a beautifully constructed book, combining personal history, the history of geology, European history, and the history of the Earth in a gradually unfolding story that reaches further and further back into geological time. The story is focused on central Italy; it starts on one of the seven hills of Rome and ends up encompassing the fascinating and complex story of the building of the Apennines.

While it’s not specifically about geological lingo, it inevitably contains a good bit of it; this includes the lovely Italian terms for some of the rocks of central Italy, for example, Scaglia rossa (a pinkish limestone tinted by iron oxides). Geologists call the limestone quarried near Assisi the Scaglia, meaning scale, because the rock can be shaped by flaking off small chips or scales; rossa means red. (Speaking of limestone, I live in limestone country on a different continent, so I was pleased to see the shout-out to limestone for its importance in making Earth habitable.) Each term is explained when it appears, and there’s a glossary in the back (as well as a good index), so the reader is never lost in a sea of beautiful but unfamiliar words.

I was particular intrigued with the word ignimbrite, which is the name given to rocks that solidify out of a pyroclastic flow or ash flow. (They’re also called ash-flow tuffs.) As you may recall, pyroclastic flows are among the most terrifying and deadly of volcanic phenomena; these clouds of vaporized rock and ash roll swiftly down the sides of an explosively erupting volcano, scorching all in their path. I’ve always kind of liked the French term for them, nuée ardente, which translates as glowing cloud or burning cloud. Thus, it seemed appropriate to me that the Latin roots of ignimbrite are ignis (for fire; we see this in ignite and igneous as well) and nimbus (as in cumulonimbus). These rocks originate in a cloud of fire.

Interestingly, however, other sources give the Latin root words as ignis and imber, which is a rain shower or rain cloud. (You may know the Latin word imber from its use in the name Mare Imbrium, which is generally translated as Sea of Rains or Sea of Showers.) I suppose it also makes sense that the rocks produced by a “cloud of fire” might be named for the shower that eventually comes out of the cloud. I looked up the 1935 paper in which Peter Marshall, a New Zealand geologist, evidently first proposed the term ignimbrite; luckily, the paper was available online, but in it he didn’t explain his thinking when he coined the word. So I will leave it there. If there are any geologists, historians of science, or etymologists who are reading this and can share any stories about the origins of this word, please leave a comment.

I knew about the hippocampus in the brain, but until I started reading The Darwinian Tourist: Viewing the World Through Evolutionary Eyes, by Christopher Wills (find in library), I didn’t know that seahorses are in the genus Hippocampus. One of the things I’m enjoying about Wills’s excellent book is that he usually lists the scientific names of the living things that appear in the illustrations. The pygmy seahorse Hippocampus bargibanti, a tiny warty creature, shows up on page 17.¹ (The warts help them blend in with the bulbous sea fan [similar to a coral] on which they live.)

The hippocampus in the brain is a structure that’s involved in forming new memories and in spatial navigation. (Actually, it’s a pair of structures, one on each side.) I knew the Greek root hippos (horse), but it had never before occurred to me to wonder if the horse had anything to do with the hippocampus. In fact, it does. In shape the hippocampus resembles the seahorse, and the scientific name Hippocampus is linked to the seahorse’s resemblance to the terrestrial horse.

Other than its head, the seahorse doesn’t resemble a horse at all. The source of the other half of the name Hippocampus, the Greek root kampos, is generally translated as sea monster. In Greek mythology, the Hippokampoi were large creatures with the front end of a horse and the back end of a fish; they provided transportation for the Nereids and pulled Poseidon’s chariot. However, kampos may be related to kampe, or caterpillar, so you could also imagine seahorses as a horse/caterpillar cross, although that’s not quite as appealing a notion.

You also see the root hippo in hippopotamus, or river horse. An 18th century anatomist, evidently in a state of confusion, referred to the hippocampus as the hippopotamus, and this confusion persisted for some time afterward. I don’t know why the thought of a hippopotamus in the brain is funnier than the thought of a seahorse in the brain, but it is.

1 Unfortunately, Wills gives its name as Hippocampus bargobanti; let’s hope that will be fixed in the Kindle edition and future paper editions.^↩