912 



HISTORICAL GEOLOGY. 



The Miocene of the Atlantic border lias afforded remains of many- 

 Cetaceans. Among them are various Dolphins, several species of Whales 

 of the genus Squalodon, related in teeth to the Zeuglodon, the largest 

 about 30 feet long. Others having the teeth excessive in number, or multi- 

 plicate, and provided with only one root ; others having similar teeth, but 

 only in the upper jaw, as in the genus Pliyseter, or that including the Sperm 

 Whale ; others with teeth in neither jaAv, as the Baleen or Whale-bone 

 Whales, but having several hundred plates of the so-called whale-bone, 

 growing vertically 'downward from above, with edges of fine fibers, to 

 serve, net-like, for gathering food of small Crustaceans and other species 

 from the ocean's waters. Fig. 1539 represents a restoration of a species of 



1539. 



%m^ 



Cetacean. — Cetotherium cephalus (x j'j). Kestoration by Cope. 



this kind, 30 feet long, from the Maryland Miocene, the Cetotherium cephalus 

 of Cope (1890). The head of the Baleen Whales makes about a third of 

 the length of the body. 



Pliocene. — The Blanco beds of the Llano Estacado, western Texas, in 

 the Pliocene, have afforded Cope remains of a Megalonyx, Mastodon mirificus, 

 Eqmis simjjlicidens Cope, a Camel of the genus Pliauchenia, and some other 

 species. 



The succession of forms in the feet and teeth under the Horse type 

 is illustrated by Marsh with the following diagram. The plate contains, in 

 a series of seven columns, figures of the fore foot, hind foot, lower joint of 

 the forearm (made up of the radius and ulna), the same of the leg (tibia 

 and fibula) ; and (5, 6, 7), others showing the length of the teeth and the 

 convolutions within them. Columns 1 and 2 illustrate the fact of the 

 diminishing number of toes with the progress of the Tertiary, until at last, 

 in the modern kind, only the middle or third toe remains, with, either side, 

 rudiments of the second and fourth in the form of the splint bones, while 

 the third toe has become increasingly larger and longer. In the regular series, 

 besides the genera there mentioned, EpiMppus of Marsh is an intermediate 

 genus between OroMpjnis and MesoMppus ; and Desmatippus of Scott, one 

 between MioMppus and Protohijypus. In the derivation from the Tapir-like 

 precursors, the type of the Horse became distinct when the middle toe 

 was decidedly stouter than that either side of it; and it reached its maximum 

 when this toe was the only one, and the other two were merely "splint" 

 bones. 



