66 ON THE CHELONIAN GENUS LYTOLOMA, [Feb. 19, 
respects with the one forming the subject of this memoir, and may 
be confidently referred to the same species. The associated cara- 
pace agrees with the type carapace of Chelone crassicostata in its 
thick ribs and the shortness of the anterior lateral facets of the 
neural bones ; and therefore serves to prove that Sir R. Owen was 
correct in referring the skull above described to that species. 
Further, by comparison of drawings and also of some of the 
actual specimens from the Eocene of Belgium, originally described 
by M. Dollo under the name of Pachyrhynchus gosseleti, and sub- 
sequently made the type of Erquelinnesia, and finally referred to 
Euclastes, I am convinced that the Belgian form is specifically 
identical with Lytoloma crassicostatum—a view in which I believe I 
am justified in saying M. Dollo himself concurs. The evidence for 
this identification is afforded by the similarity in the contour of the 
cranium and mandible, and by the form of the neural bones of the 
carapace, the carapace of L. planimentum (Owen) being readily 
distinguished by the equality in the length of the anterior and 
posterior lateral facets of the neurals. 
Finally the skuil of no. R. 918 has enabled me to identify with 
this species a still younger cranium in the British Museum (no. 
38954). This specimen is important as showing that in the young 
the posterior nares were situated much more anteriorly than in the 
adult; and I find that in the existing Thalassochelys there is a 
tendency as age advances for the posterior nares to recede to a 
certain extent, and also towards a gradual increase in the length of 
the mandibular symphysis. 
EXPLANATION OF THE PLATES. 
Puate VI. 
Palatal aspect of the skull of Lytoloma crassicostatum ; from the Lower 
Eocene of Harwich. Two thirds nat. size. 
B.O., basioccipital; B.S., basisphenoid; Z.0., exoccipital ; Mn., mandibular 
symphysis; Ops., opisthotic; Pa/., palatine; P., pterygoid; Q./., quadrato- 
jugal; Qu., quadrate; Sup., supra-occipital; 7.7, palatal aperture of tem- 
poral fossa; /.a., aperture of labyrinth; m.s., masseteric ridge of mandible; 
pt.n., posterior nares. 
Prate VII. 
Fig. 1. Occipital aspect of the skull figured in Plate VI. gv., slit for insertion 
of genio-hyoid and genio-glossal muscles ; s¢p., canal for stapes; f.m., 
foramen magnum. Other letters as in Plate VI. 
2. Dorsal aspect of the associated humerus. 
Both figures two thirds nat. size, 
