Ichthyosaurs and Plesiosaurs 63 



Among the ichthyopterygian, or ichthyosaurian reptiles, 

 Ichthyosaurus campylodon of the Chalk is also abundantly 

 represented by teeth and fragments of the jaws in the Cam- 

 bridge Greensand. Other names have been applied, without 

 definition, by Prof. Seeley 1 to ichthyosaurian remains from 

 the same formation in the Sedgwick Museum. A specimen 

 in the British Museum has been assigned by the present 

 writer 2 to the Jurassic genus Ophthalmosaurus, distinguished 

 from Ichthyosaurus by the articulation of three (in place of two) 

 bones to the lower extremity of the humerus and femur respec- 

 tively. The species has been named 0. cantabrigiensis. The 

 genus is allied to the toothless Baptanodon of the North 

 American Cretaceous, but was apparently furnished with teeth 

 in the front of the jaws. Finally, a thigh-bone, or femur, 

 from the Cambridge Greensand in the Sedgwick Museum 

 has been regarded by Prof. Seeley 3 as indicative of a distinct 

 generic type of ichthyosaurian, under the name of Cetarthro- 

 saurus walkeri. It is stated " to present a resemblance one 

 degree nearer to the femur of the monotremes than that of 

 Ichthyosaurus" 



Teeth and other, remains of the second great group of 

 marine Secondary reptiles, the Sauropterygia, or Plesiosauria, 

 are likewise of common occurrence in the Cambridge Green- 

 sand, although but few of the species have been satisfactorily 

 defined. Among the most abundant of these are the large 

 deeply-fluted conical teeth of the large-headed and short- 

 necked Cretaceous genus Polyptychodon, of which the two 

 species P. continuus and P. interruptus have been recognized 

 from this formation. Of the more slenderly built and longer- 

 necked types included in the genus Cimoliosaurus, specimens 

 referable to C. bernardi of the Chalk have been recognized; 

 while several undefined types, such as the so-called Plesio- 

 saurus planus and P. cantabrigiensis of Seeley, probably 

 belong to the same or a nearly allied genus. 



1 "Index to Ornithosauria, etc." xvn. 



2 Cat. Foss. Rept. Brit. Mus. n. 9 (1889). 



8 Quart. Journ. Geol.Soc. xxix. 505 (1873). 



