| OCR Text |
Show 66 ON THE CHELONIAN GENUS LVTOLOMA. [Feb. 19, respects with the one forming the subject of this memoir, and may be confidently referred to the same species. The associated carapace 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 subsequently made the type of Ercpxelinnesia, 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 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. PLATE 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; E.O., exoccipital; Mn., mandibular symphysis; Ops., opisthotic; Pal., palatine; Pt., pterygoid; Q.J., quadratojugal; Qu., quadrate; Sup., supra-occipital; T.F., palatal aperture of temporal fossa; l.a., aperture of labyrinth; m.s., masseteric ridge of mandible; pt.ii., posterior nares. P L A T E VII. Fig. 1. Occipital aspect of the skull figured in Plate VI. gn., slit for insertion of geniodiyoid and genio-glossal muscles ; stp., canal for stapes; /.m., foramen magnum. Other letters as in Plate VI. 2. Dorsal aspect of the associated humerus. Both figures two thirds nat. size. |
