The affinities of the Sphargidae and their position in the system are still debatable. Whilst some authorities, _e.g._ Cope, Dollo, and Boulenger look upon _Sphargis_ as the sole remnant of a primitive group in opposition to all the other recent Chelonia, Baur considered it the most specialised descendant of the Chelonidae. Dames agreed with him. Van Bemmelen has modified this view in so far as he regards _Sphargis_ as the most specialised Chelonian, but considers the differences between it and the Chelonidae great enough to conclude that both Sphargidae and Chelonidae represent two independent, partly parallel, branches which have arisen from two different groups of terrestrial tortoises. Case,[130] from the study of _Protostega_ and other fossil forms, tends towards Baur's view. He believes that _Sphargis_ is the culminating form of a branch which through _Psephophorus_ and with _Eosphargis_ has sprung from some creature like _Lytoloma_, which at the same time is the starting-point of another branch which culminates in the genera _Thalassochelys_ and _Chelone_, while lastly a third branch contains _Protostega_, _Protosphargis_, and _Pseudosphargis_. In other words, he considers them all Chelonidae. If he is right we have of course no business to separate _Sphargis_ with its fossil allies from the rest of the Chelonia as "Athecae." However, Case has not proved his point. It is easy enough to understand that the characters of the cranium and plastron of _Sphargis_ are in a condition which by partial reduction can be derived from that of typical Chelonidae. The structure of the {337}cervical vertebrae, the absence of the marginal plates and the peculiar articulation of the nuchal with the last cervical vertebra can be explained as convergent analogies, just like the paddles of _Carettochelys_. But the shell of _Sphargis_ is fundamentally different from and not homologous with that of the others. Cope was therefore quite justified in distinguishing 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 θήκη or shell. Both authors meant, however, by Theca the epidermal shields, but even this distinction is rendered invalid by _Carettochelys_.