8 SPHARGID&, 
The cervicals are short and similar to those of the Chelonide. All 
the dorsal ribs except the last articulate with the neural arch and 
two centra; the first and tenth ribs are short. The caudal vertebra 
are proceelous, Chevron bones are absent. 
The dorsal shield consists of a mosaic of extremely numerous 
polygonal bony plates of unequal size, the largest forming seven 
longitudinal keels or ridges ; it is produced into a pointed supra- 
caudal portion, which covers the tail. In the young the dorsal 
shield is soft and leathery. Between the shield and the last cervical 
vertebra a bone is present which is the homologue of the nuchal of 
other Chelonians. 
The ventral shield is similar to and continuous with the dorsal, 
and likewise bears ridges of enlarged tubercles; in the young both 
shields are extremely similar, but, with age, they differ in the ventral 
remaining soft, only the tubercles of the longitudinal ridges and 
afew others being bony. There is an underlying annulus of slender 
bones, eight in number—the plastral elements—viz. the epiplastra, 
the hyoplastra, the hypoplastra, and the xiphiplastra; an ento- 
plastron or interclayicle is absent. 
The skull bears a certain resemblance to that of Chelone, which 
is, however, limited to the shape and the general constitution of the 
temporal roof; in the absence of the column-like processes of the 
parietals descending to the pterygoids in front of the supraoccipital 
and the prootics, and in the opisthotic not coming in contact with 
the squamosal, it differs from that of all other Chelonians. 
The external bony nostril is very large and situated entirely upon 
the upper surface of the skull, in front of the orbits; the choane 
are quite anterior, the nasal duct being nearly vertical, and are 
separated by the vomer, which also separates the palatines. The 
premaxillaries are distinct; the prefrontals, which join the post- 
frontals, form a very short suture in front of the frontals. In one 
of the adult skulls in the British Museum, the lachrymals are nearly 
completely separated from the maxillaries; but this is merely an 
individual peculiarity. The parietals entirely or nearly entirely 
cover the supraoccipital; they form a long suture with the post- 
frontals, and a short one with the squamosals. The lower border 
of the postfrontal joins the jugal and the squamosal, and, contrary 
to what exists in the Chelonide, is separated from the quadratojugal 
by the two latter bones. The quadrate bone is a good deal within 
the outer border of the skull, and separates the opisthotic from 
the squamosal; the ear-chambers are open behind. The foramen 
magnum, which is deeper than broad, is bordered by the basi- as 
well as by the supra- and exoccipitals. The pterygoids are smaller 
than in any other Chelonian; they are nearly entirely separated 
from each other by the large basisphenoid, which extends forwards 
as far as the posterior borders of the orbits, and widely separated 
from the maxillaries by the palatines ; their lateral borders are 
nearly straight, and without “ectopterygoid” processes; in front, 
they are in contact with the large palatines, behind with the 
quadrate and the basioccipital. The mandible has the same struc- 
