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Show 1889.] CHELONIAN GENUS LYTOLOMA. 63 the latter differed from all living Turtles in the peculiar flatness and the great relative width and length of the mandibular symphysis, of which only the inferior surface was displayed. It was also shown that the cranium differed in the upward instead of vertical direction of the orbits, and by the narrower interorbital bar. The palate being concealed in both skulls, nothing could of course be said regarding it. In the figure of Chelone crassicostata it was, however, indicated that the nasals were separate from the prefrontals; but an examination of the specimen shows that there is no possibility of determining this point ; but in C. planimentum they are certainly united, as in other Cryptodirans. Subsequently Prof. Seeley, in establishing the genus Glossochelys, relied not only on the distinctive features pointed out by Sir Richard Owen, but also on the remarkably large size of the hyoids, which are preserved in G. planimentum, this feature affording the grounds on which the generic name was chosen. Prof. Cope, in the memoir cited, was enabled to give fuller characters from the evidence of the mandible described as Lytoloma; while the cranium figured as Euclastes agreed in the characters of the orbits and adjacent regions with the English specimens. Important evidence was also adduced as to the nature of the limb-bones and the shell in this or allied types. Thus the humerus was shown to differ somewhat from that of existing Turtles, and was said to approximate to that of Chelydridee. In the shell the ossification was demonstrated to be more complete than in Thalassochelys, while its xiphiplastral elements were relatively wider and united in the middle line throughout their length. So impressed, indeed, was the Professor with the distinctive features of the group, that he regarded them as constituting a distinct family under the name of Pro-pleuridee. lt was reserved, however, for M . Dollo to throw more definite light on the cranial structure of Lytoloma, and in the memoir of 1886, to which allusion has been already made, he showed that not only was the oral surface of the palate and mandibular symphysis devoid of ridges and remarkable for its extreme flatness, but also that the cranium was at once distinguished from that of all existing forms by the extremely backward position of the posterior nares, which were situated in the posterior third, instead of the anterior half of the cranium. It was stated at the same time that the nasals are distinct from the prsefrontals, but no mention is made whether this statement rests upon Sir Richard Owen's figure of Lytoloma crassicostatum, or as the result of actual observation of the Belgian specimens. In this memoir it was concluded that the creation of a separate family for this group of Turtles was not justified, and that the Propleuridee of the American palaeontologist was not entitled to rank as more than a subfamily of Chelonidee. In the paper published in 1887, and already quoted, M . Dollo comes, however, to the opposite conclusion, considering that the marked difference of the humerus of Lytoloma (Euclastes) from that of Chelone is sufficient to indicate a |