66 ON THE CHELONIAN GENtJS 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 fecets 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 Pachyrhunchus gosseleti, and sub- 

 sequently made the type of Erquelinnesiu, 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 skull 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 Thalassochehjs there is a 

 tendency as age advances for the posterior nares to recede to a 

 certain extent, and also towards a gradual increase iu the length of 

 the mandibular symphysis. 



EXPLANATION OF THE PLATES. 



Plate VI. 



Palatal as^^ect of the skull of Lytoloma crassicostatum ; from the Lower 

 Eocene of Harwich. Two thirds nat. size. 



i?.0., basioreipital; B.8., basisphenoid ; £'. 0., exoccipital ; iI/»., mandibular 

 symphysis; Ops., opistholic; Pal., palatine; Pf., pterygoid; Q.J., quadrato- 

 .jugal; Qn., quadrate; Sup., supi-a-occipital ; T.F., palatal aperture of tem- 

 poral fossa; l.a., aperture of labyrinth; m.s., masseteric ridge of maudible; 

 pt.n., posterior nares. 



Plate VII. 



Fig. 1. Occipital aspect of tlie skull figured in Plate VI. gn., slit for insertion 

 of genio-hyoid and genio-glussal muscles; sfj)., caual for stapes; f.m., 

 foramen magnum. Other letters as in Plate VI. 

 2. Dorsal aspect of tbe associated humerus. 



Both figures two thirds nat. size. 



