314 D. WS. Watson—Chelonian from the Purbeck, Swanage. 
Thalassemys and transfers it to the Amphichelydia or Pleurodira. The 
occurrence of infra-marginals at once puts the Pleurodira out of court. 
Hay, in his recent monograph of the fossil turtles of North 
America, published by the Carnegie Institution, retains two families 
in the Amphichelydia, the Pleurosternide and the Baénide. The 
only differences in the shells of the families are: (1) The Baénide 
have more strongly developed axillary and inguinal buttresses than 
the Pleurosternide; (2) in the Baénide the mesoplastra contract 
strongly as they approach the middle line, whereas in the Pleuro- 
sternide they retain their full width until they meet. In our species 
the mesoplastra agree with the condition characteristic of the 
Pleurosternide.. 
The buttresses cannot be well observed in the Manchester specimens, 
but judging from the inner aspect of the distal end of the left 
hyoplastron, which is visible in L. 9520, the axillary buttress must 
have been small and feeble. Therefore in this character also there 
is agreement with the Pleurosternide. The animal under discussion 
differs from Pleurosternum itself in the presence of a nuchal shield 
and in the emarginate anterior edge. 
The family only contains two other described genera, Helochelys 
aud Glyptops. Helochelys differs markedly from our form in the 
very loose connexion between the plastron and carapace, and there 
only remains for comparison Gilyptops. 
This genus was founded by Marsh for a medium-sized Chelonian 
from the Morrison, Como, or Aflantosaurus beds of the United States 
Upper Jurassic. The type species, G. ornatus, is held by Baur to 
be specifically identical with Compsemys plicatulus of Cope. Hay has 
shown that this species does not belong to the genus Compsemys, but 
represents a very distinct genus for which Marsh’s name Glyptops 
must be adopted. Judging from Hay’s beautiful figures and excellent 
description, G. plicatulus is extremely similar to our animal: with 
two exceptions the two agree exactly. The form of the carapace and 
plastron is similar, the position of the epidermal shields and the 
ornament are unusually alike; a nuchal shield is said to occur in 
G. plicatulus by Baur. The only differences in fact are these: (1) In 
our turtle there is sometimes a preneural bone. (2) There are 
two suprapygals in the English specimens; one of Hay’s specimens, 
the original of his plate vi, is stated to have only one, but his photo- 
graphic plate shows a crack or suture in a position very similar to 
that separating the first from the second suprapygal in my specimens. 
(3) In our turtle the xiphiplastra are rather more deeply notched 
than in G. plicatulus. 
The only one of these differences that is at all serious is the first, 
and as it appears that this difference is liable to occur between 
individuals of the same species, it can hardly be held to be of generic 
importance. It thus appears that 7’alassemys ruetimeyert, Lyd., 
should be referred to the genus Glypiops, and in future be known 
as Glyptops ruetimeyeri (Lyd.). It deserves to be noticed that Glyptops 
has a large range in time from the Como Beds of the Upper Jurassic 
to the Denver Beds at top of the Upper Cretaceous; its large range 
in space is thus rendered much less unhkely. 
en 
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