CRETACEOUS PERIOD. 545 



rotomaria, and the Mesozoic Nerintza. Towards the close of the 

 Cretaceous period, we meet for the first time with Gasteropods 

 of the existing genera Valuta, Mitra, Cypraa, Fasdolaria, 

 Strombus, &c. 



The most characteristic Molluscs of the Cretaceous period 

 are Cephalopods. The Dibranchiate section of this order is 

 represented by species of Belemnites itself and by the genus 

 Belemnitella. Of the Tetrabranchiates we find species of the 

 old genus Nautilus ; but this section is represented mainly by 

 complex and beautiful forms of the Ammonitida. The genus 

 Ammonites itself, dating its existence from the Upper Trias, is 

 represented by many Cretaceous species, and finally disappears 

 with the close of this period. Ancyloceras dates from the 

 commencement of the Jurassic period, and also dies out in the 

 Chalk. Finally, the AmmonitidcB are represented by the genera 

 Baculites, Turrilites, Scaphites, Hamites, Ptychoceras, Toxoceras, 

 and Crioceras, which make their first appearance in the Creta- 

 ceous rocks, but which are not known at present to occur in 

 any later deposit. 



Remains of Fishes are by no means rare in the Cretaceous 

 rocks. Teleostean Fishes appear here, and are well represented 

 by forms more or less allied to existing types (Beryx, Osmero- 

 ides, &c.) Ganoids (such as Lepidotus, Caturus, Pycnodus, &c.) 

 are plentiful ; but are of little special importance. Of the 

 Cestracionts, we have the old genus Acrodus, and the Creta- 

 ceous genus Ptychodus. Hybodonts also occur, and teeth of 

 true Selachians (Lamna, Carcharias, Odontaspis, &c.) are not 

 wanting. 



The Reptiles of the Cretaceous belong mostly to the orders 

 of the Pterosauria, Ichthyopterygia, Sauropterygia, and Deino- 

 sauria all of which die out with the Cretaceous period. The 

 best known of the Deinosaurs is Iguanodon, which is confined 

 exclusively to the Lower Cretaceous period, but the genera 

 Ichthyosaurus, Plesiosaurus, and Pterodactylus, extend their 

 range into the Upper Cretaceous. The only exclusively Cre- 

 taceous group of Reptiles is that of the Mosasauroids, which 

 are known to have existed up to the last phase of this period 

 (Maestricht beds). The Mosasauroids are found in Europe, 

 but more abundantly in North America ; and the recent dis- 

 covery by Professor Marsh, that the body was, in some cases, 

 furnished with a covering of bony scutes, would seem to re- 

 move them from the Lizards, amongst which they have been 

 generally placed. 



Birds have not been shown with certainty to have existed 

 during the Cretaceous period in Europe, though various re- 



2 M 



