IX SPHARGIDAE 337 



cervical vertebrae, the absence of tlie marginal plates and the 

 peculiar articulation of tlie nuchal with the last cervical vertebra 

 can be explained as convergent analogies, just like the paddles of 

 CarettocheJys. But the shell of Sphargis is fundamentally different 

 from and not homologous with that of the others. Cope was 

 therefore quite justified in distinguisliing the Sphargidae as 

 "Athecae" in opposition to the others which Dollo later on, by 

 contrast, named " Thecophora." Unfortunate names, since both 

 groups are undeniably in possession of a Ot'jKr] or shell. Both 

 authors meant, however, by Theca the epidermal shields, but even 

 this distinction is rendered invalid by Carettochelys. 



The most reasonable explanation has been suggested by Hay.^ 

 The mosaic polygonal components of the shell of Sphargis are, 

 so to speak, an earlier generation of osteodermal plates than the 

 later generation of longer and broader bony plates which in the 

 Thecophora come into contact, and fuse with, the neural arches 

 and ribs. The osteoderms of Sphargis belong to the same 

 category as tlie dermal ossifications in the scutes of Crocodilia, 

 whilst the plates of the carapace and plastron of the Tliecophora 

 belong to the category of the abdominal rilis. Sphargis has the 

 first kind in its peculiar shell, the second kind in the deeper 

 lying plastron and in its neural plate. But it has lost, or 

 perhaps had never developed, the horny shields. The only 

 difficulty is, however, the presence of a plastron and of a typical 

 neural plate in Sphargis. This difficulty is not very serious. 

 The plastron is a very old institution. It occurs together with 

 the more superficial osteoderms in Caiman, and the nuchal plate 

 may be the oldest of all dorsals. We can scarcely imagine that 

 the direct ancestors of Sphargis had developed both kinds of 

 shells, and that comparatively recently the inner shell of the 

 carapace was lost, leaving only the nuchal plate. Fossils do not 

 support such an assumption. Undoubted ancestral forms of 

 Sphargis are very rare. Psephop>horus of the Oligocene and Mio- 

 cene of Europe had a continuous mosaic shell much resembling 

 that of Sphargis ; Ensphargis is represented by a well-preserved 

 skull from the London clay. Then follows a wide gap until we 

 come to Psephoderma of the Ehaetic, or Upper Trias of Bavaria ; 

 the large fragment of whose dorsal shell is composed of about 200 

 mosaic pieces. If this fragment really formed part of the shell 



1 Amer. Natural, xxxii. 1898, p. 929. 

 VOL. vin Z 



