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 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 Huclastes 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 Chelydride. In the shell the ossification 
was demonstrated to be more complete than in Z'halassochelys, 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- 
pleuride. 
It 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 preefrontals, but no mention is made 
whether this statement rests upon Sir Richard Owen’s figure of 
Lyioloma 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 
Propleuride ot the American paleontologist was not entitled to rank 
as more than a subfamily of Chelonide. 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 (Luclastes) trom that of Chelone is sufficient to indicate a 
