MR. D. M. 8S. WATSON ON THE CHELONTA. 1011 
93. Hunotosaurus africanus Seeley, and the Ancestry of 
the Chelonia. By D. M. 8. Watson, M.Sc., F.Z.S., 
Lecturer on Vertebrate Paleontology in University 
College, London. 
[Received June 3, 1914: Read November 24, 1914.] 
(Plate VII.* & Text-figure 1.) 
INDEX. 
Page 
Origin and Structure of the Chelomia ........................... 1011 
Description of Skeleton of Hunotosaurus ..................... 1015 
Relationship of Hunotosaurus to the Chelonia ............... 1019 
The great group of the Chelonia is of unusual interest, because 
it is the only example of a persistent and world-wide Order whose 
structure is entirely dependent on a bizarre specialization : the 
development of the shell. 
Tortoises are first known from the Upper Trias of Germany, 
where they are represented by extremely typical and apparently 
not markedly primitive forms. 
By an analysis of the structures common to all Chelonia, it 
is possible to form some idea of the chief features which must 
have been present in the ancestors of the group. Although 
it is impossible to determine whether or not these characters 
were actually all present together in any type, it will be con- 
venient to add them together to form an imaginary animal 
which may be called ‘ Archichelone.” 
Skull_—The skull of the Chelonia has been discussed by many 
authors, most recently by Hay, whose conclusion is adopted: in 
this paper. 
Since the description of Sphenodon by Dr. Giinther, it has been 
repeatedly pointed out that, in such a type as Chelone with a 
completely roofed skull, the four bones—post-orbital, squamosal, 
jugal, and quadratojugal—which form the lateral temporal arcades 
of Sphenodon are present. If I do not misinterpret their opinions, 
most of those who have discussed the subject believe that the 
skull of Chelone has actually arisen from a two-arched form by a 
secondary enlargement of these bones, until they unite by their 
edges. This conclusion is rendered improbable by the fact that 
some Cotylosaurs, Labidosaurus and Pariasaurus for example, 
whose skulls have certainly always been completely roofed, have 
an identical arrangement of bones in the temporal region. 
' In many Chelonia, apparently without any correlation with the 
animal’s habits other than the power of retracting the neck, the 
temporal region is open; but this opening never results in 
the formation of a two-arched form—in all-cases it has been 
* Por explanation of the Plate see p. 1020. 
68* 
