ZEUGLODONT AND OF LEATHERY TURTLE. 



!15 



The plates filling the iiitei-vals between the ridges are smaller 

 than the keeled plates and of more irregular forms (PI. II. fig. 2). 

 So far as can be seen, between each pair of ridges there are only 

 two I'ows of these plates, except that here and there a small irre- 

 gular plate yany be intercalated. This comparatively' small number 

 of intermediate plates seems to be a primitive character, since in 

 Fsejihophorus and Dermochelys there is a progressive increase in 

 their numbers: in Pse'phophorus there were at least six rows and 

 in Dermochelys they are very numerous. The epithecal plates 

 are very thick and massive ; in the median row the thickness of 

 the lateral portions is about 10 mm., while at the ridge it may be 

 as much as 16 mm. In the lateral plates above noticed, the 

 general thickness has decreased to 7 mm. or, at tlie ridge, 9 mm. 

 According to Seeley *, the plates of Pse2)hophorus may attain a 

 thickness of nearly 10 mm., but are usually thinner. The outer 

 surface of the plates is beautifully sculptured, with a, series of 

 iriegular tuberosities in the middle bordered by I'idges more or 

 less radially arranged, and running to the margin which is usually 

 bevelled ofl", so that the suture between neighbouring plates runs 

 at the bottom of a groove. In Psej^hophorus also the surface of 

 the plates is sculptured, but the ornament appears to consist 

 of ladial ridges only and to be less pronounced than here. 

 Probably in life the outer surface was covered by a leathery skin, 

 or possibly by horny plates, though this does not seem likely. 

 The presence of this strongly marked ornament seems to show 

 that this Turtle Avas not adapted for rapid motion through the 

 water, but was probably a littoral or even a swamp-living form 

 like Trionyx. It is unfortunate that nothing is known of the 

 plasti'on. 



Text-fio-ui-e 2. 



Transverse seotioii tlivouijh epithecal and thecal shell of Cosmochclj/s. 



cosif.. costal plate; c^j/., epithecal shell; l.r., lateral ridge of ditto; )».r., iiiediani 

 ridg-e of ditto ; »., neural plate; }>.«., neural arch ; w.s., neural spine; ?•., rib. 

 1 r.at. size. 



The thecal skeleton, so far as preserved, consists of the remnants 

 of the costals and neurals. The costal plates are confined to the 

 upper end of the ribs, where they are fairly well developed,. 



* Seele3", " Note on PsepJiopJinni.t pohigonns v. Me_yer." Quart. .Tourn. Geol. Soo 

 vol. xxxvi. (1880), p. 406. 



