CRETACEOUS FAUNA. 169 



the scallops (Pecten), and the genus Inoceramus among the Hetero- 

 myaria, are distinguished by their numbers ; but the most charac- 

 teristic elements of the lamellibranch fauna are furnished by two 

 families of very inequivalve-shelled mollusks, the Chamidse, with 

 the genera Requienia, Monopleura, Caprina, and Caprotina, and 

 the so long misunderstood Rudistse (Sphaerulites, Radiolites, and 

 Hippurites), whose forms so eminently characterise the southern 

 belt of European and American Cretaceous deposits, and which ap- 

 pear and disappear with this period. The siphonate univalves have 

 an almost exclusively modern asjject, and comprise among others 

 representatives of the families Fusidae, Strombidse, Muricidae, Tri- 

 tonidae, Buccinidae, Cancellariidae, Pleurotomidae, Conidae, Olividae, 

 and Cypraeidae. 



Turning to the vertebrates, we find in the lowest class, Pisces, 

 the introduction for the first time of teleosts, or true bony fishes, 

 that ichthyic group which at the present day surpasses, both in 

 individual members and variety, all the other orders of fishes put 

 together. These earliest teleosts, although not very abundant, 

 comprise a considerable number of modern types (Clupea, Esox, 

 Osmerus, Beryx) ; but it is not till the Tertiary period that they 

 acquire any well-marked development. No amphibian remains 

 have been detected in any Cretaceous deposit. Reptiles, on the 

 other hand, are exceedingly abundant, and comprise most of the 

 types whose existence has been indicated in the Jurassic seas. 

 Thus, of the modern groups, we have turtles, lizards, and croco- 

 diles (of both the amphiccelous — Hyposaurus — and procoelous tj'pes 

 — Holops, Gavialis), and, in addition, the first true serpent (Si- 

 moliophis). The extinct orders Ichthyosauria and Plesiosauria 

 are still represented, and in Elasmosaurus, belonging to the lat- 

 ter, we meet with one of the most formidable types of the finned 

 Reptilia. Here, also, are found some of the most gigantic of the 

 Dinosauria — Iguanodon, Megalosaurus, Hadrosaurus, Camarasau- 

 rus — and the remarkable group of the Pythonomorpha, or "sea- 

 serpents " — Mosasanrus, Leiodon, Clidastes — which in several re- 

 spects united the characters of both serpents and lizards. The 

 largest of the pterodactyls, or flying reptiles, having an expanse 

 of wing of from twenty to twenty-five feet, or even more, occur 

 in deposits of this period, and are represented by the normal- 

 toothed types, and by such, as the American Pteranodon, in which 



