FROM THE LOWER, LIAS OF CHARMOTJTH". 



467 



similar bone are indicated near the same spot. Their presence can 

 scarcely be accidental ; and they may possibly be dermal plates. 

 That they are not found elsewhere would simply point to the 

 restricted distribution of dermal scutes in the species, they having 

 originally been present in the pelvic region and nowhere else. 

 The thin film, however, has nothing of the nature of scales and 

 scutes, so far as we can see ; it was a continuous membrane, not a 

 collection of separate individual structures. It can easily be 

 detached from the underlying surface, owing, it would seem, to the 

 presence of a thin whitish layer, apparently calcite, which is more 

 strongly adherent to the film than to the surface beneath. 



The surface of this film is variously marked ; but all the different 

 markings may be described as essentially of the nature of 

 wrinkles. In the film of the dorsal band they have the appear- 

 ance of fine regular rounded ridges, giving the surface a re- 

 semblance in some degree to " corded silk ; " elsewhere, as over the 

 bases of some of the neural spines, the ridges lose to a great 

 extent their straightness and regularity, take a tortuous course, 

 though generally with one prevailing direction, and are more appa- 

 rently mere wrinkles ; but over the greater part of its extent an 

 additional feature presents itself in the form of long, fine, parallel 

 grooves, bordered by fine ridge-like margins, and looking as though 

 they had been scored by a fine needle: they vary in distance from 

 each other ; but the best-marked are about inch apart. They main- 

 tain one general direction from before backwards on the bodies of 

 the vertebrae, the exposed outer sides of the ribs, and on the stone 

 between them, from the forty-second to the fiftieth vertebra. 

 Between the grooves minute wrinkles are abundant, mostly undu- 

 lating, sometimes straight, not always confined to the space between 

 two grooves, but sometimes crossing them without changing their 

 course ; they are inclined at all angles to the grooves, but are 

 chiefly transverse to them. 



What the precise nature of this film may be is by no means 

 clear. Prom its distribution one might infer that it originally 

 formed a part of the integumentary investment. It closely resem- 

 bles in character the structure which has been regarded by Mr. 

 Moore as forming a part of the integument of IeJithyosaurus, and 

 which this acute observer has compared to the wrinkled surface of 

 the skin of the Porpoise *. The resemblance between this surface, 

 as seen in our museum-specimens, and that of the investing film 

 in Plesiosaurus is, indeed, great ; and if such a skin were capable of 

 fossilization, one might fairly allow that Plesiosaurus had been 

 invested with it. It is very certain that the film in our fossil 

 specimen was of a yielding flexible nature, or it could not have so 

 neatly covered the exterior of the ribs and adapted itself to the 

 ends of the transverse processes and the angles between the neural 

 spines and the vertebral bodies as it has done. 



* Som. Archseol. & Nat. Hist. Soc. Proc. 186-566, p. 179. 



