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 interior 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 

 determniing this point ; but in C. planimentum they are certainly 

 united, as in other Cryptodirans. 



Subsequently Prof. Seeley, in establishing the genus Glossochelijs, 

 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 C. planimentum, this feature affording the 

 grounds on which the generic name was chosen. 



Prof. Cope, in the memoir cited, was enabled to give fuller charac- 

 ters from the evidence of the mandible described as Lytoloma ; 

 while the cranium figured as Eudastes 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 I'rom that ot existing Turtles, and was said 

 to apjjroximate to that of Cheli/dridce. 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- 

 pleuridce. 



It was reserved, however, for M. JJollo to throw more definite 

 light on the cranial structure of Lijtoloma, and in the memoir of 



1886, to which allusion has been already made, he showed that not 

 only was the oral surface ot 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 

 Propleuridce ot the American palaeontologist was not entitled to rank 

 as more than a subfamily of Ghelonidce. In the paper published in 



1887, and already quoted, M. Dolio comes, however, to the opposite 

 conclusion, considering that the marked difference of the humerus of 

 Lytoloma {Eudastes) from that of Chelone is sufficient to indicate a 



