Ckelone 



gigas. 



Chelone mydas, 



inches. 



lines. 



inches. 



lines. 



IS 1 







10 







14 s 



6 



7 







13 



6 



6 



3 



5 



3 



2 



5 



3 



9 



2 



7 



2 



10 



1 



1 



4 







1 



5 



FOSSIL REPTILIA OF THE LONDON CLAY. 



Length .... 



Breadth .... 



Breadth across outside of tympanic articular end 

 Antero-posterior extent of outlet of orbit 

 Vertical extent of outlet of orbit 

 Height of nostril 

 Breadth of nostril 



Notwithstanding an obvious flattening vertically with some distortion I incline to 

 regard the skull of Chelone gigas as of relatively less vertical extent than that of Chelone 

 mydas. The orbit is relatively larger and is longer in proportion to its vertical 

 diameter. The slight downward extension of the upper border is near the fore part of 

 the cavity in Chelone gigas, near the hind part in Chelone mydas. The nostril in 

 Chelone gigas is broader in proportion to its depth, more especially towards its base. 



The subtrenchant alveolar border of the maxillary is continued from beneath the 

 orbit in an uninterrupted feebly convex line to the premaxillaries. There is no indication 

 of the abrupt angular notch which produces the tooth-like process of the maxillary 

 anterior to the orbit in Sphargis. 



So far as the sutures can be traced on the upper expanse of the cranium (Plate I) 

 they conform in the main with those of Chelone mydas. 



The vaulted cavities roofed over by the parietals and mastoids are relatively lower 

 and smaller in Chelone gigas than in Chel. mydas, and this irrespective of the degree of 

 pressure which has somewhat affected the shape of the hinder outlets. 



The superoccipital spine seems from the size of its broken base to have had the same 

 relative degree of production beyond the foramen magnum in the gigantic fossil as in the 

 recent Turtle. 



The bilamellar base of the superoccipital element forming the keystone of the arch of 

 the myelonal outlet has been slightly dislocated and pressed downward into the foramen 

 magnum ; but the exoccipitals retain their natural position as the side walls of that 

 orifice. The part of the 'foramen magnum' (PL II, fig. 1) contributed by the basi- 

 occipital is less prominent, more extensively concave, than in Chelone mydas, in which 

 such concavity is represented by a central pit. The whole condyle is broader in propor- 

 tion to its depth, and the basioccipital tract anterior to the condyle is relatively broader 

 than in Chel. mydas. 



So much as could be exposed by the mason's chisel of the palatal surface of the 

 enormous cranium (PL II, fig. 3) accorded in the main with the configuration of the 



1 Part of the superoccipital spine has been broken away. 



2 Slightly increased by posthumous flattening. 



