Sauropterygia. 45 
and compared, is from the London Clay of Sheppey, and 
represents a still larger form related to the living Leathery 
Turtle. These were true marine turtles, like the “ Logger- 
head ” Turtle of the present day (Fig. 59). 
Fig. 60. — Fragment ot Carapace of Psephoderma alpinum , Meyer; Trias, Bavaiia 
nat. size). 
The oldest Chelonians known are the Ghelytherium obscurum, 
Meyer, from the Triassic sandstones, Stuttgart ; and the 
Psephoderma alpinum, Meyer, from the Lower Keuper of 
Hoheneck ; and P. anglicum , Meyer, from the fthsetic of Bristol. 
Of the fifty-two genera and one hundred and thirty-one 
species or varieties of fossil Chelonians named in the collection, 
only eighteen genera and ten species can be with certainty 
correlated with living forms ; whilst for a few of the more 
remarkable extinct forms, such as Miolania , Pelobatochelys , and 
Psephoderma , special families have been constituted for their 
reception. 
Order IX. — S AUROPTERY GIA. 
In this extinct order the body has no exoskeleton ; the 
neck is more or less elongated, and the tail short. In the skull 
the nares, or nostrils, are lateral and placed near the orbits. 
The premaxillae are very large, and there is a well-developed 
parietal foramen in the adult. The symphysis of the mandible 
is united by a suture (Fig. 63). The teeth are implanted in 
distinct sockets and confined to the margins of the jaws ; they 
have curved sharp crowns with fluted enamel. Each rib articu- 
lates to a single vertebra, the facets for the cervical ribs may be 
either single or double, and are situated entirely on the 
centrum. The vertebrae are amphicoelous (concave at both 
ends). The neck may have as many as from 21 to 40 vertebrae. 
A few of the vertebrae behind the cervicals have the ribs arti- 
culating partly on the arch and partly on the centrum : these 
have been named pectoral vertebrae. The ribs attached to the 
dorsal vertebrae have the articulation entirely on the arch, 
which generally forms an elongated transverse process. The 
Wall-case, 
No. 12. 
Table-case, 
No. 22. 
Wall-cases, 
9, 10, 13. 
Table-cases, 
15, 16,P17. 
