575 
relative breadth of the vertebral scutes; which are nearly twite as 
broad as they are long. - 
The céntral vacuity of the plastron is subcircular, ard, as might 
be expected, from the apparent nonage of the specimen, is wider 
thafi in the Chel. longiceps ; but the toothed processes given off from 
the ifner tiargin of both hyo- and hyposternals aré small, sub- 
equal, regular in their direction, and thus resemble those of thie 
Chel. longiceps. 
The length of the expanded part of the third rib is oné inch seveii 
lines ; its aiitero-posterior diameter or breadth, six lines; in the form 
of the vertebral extremities of the ribs and of the vertebral plates to 
which they até articulated, the present fossil résembles the Chel. 
aad 
‘hé author knows of no recent example, however, of the Cheloiie 
that offers Such varieties in the form of its epidermal scutes as would 
warrant the present Chelonite being considered a variety merely 
of the Chel. longiceps ; and he therefore indicates the distinct species 
Which it seéins to represent, by its main distinctive character, under 
the name of Chelone latisculata. 
4. Cheloné convera.—The fourth species of Chelone, indicated by 
a nearly coriplete cuirass, from Sheppey, holds a somewhat inter- 
mediate position between the C. bréviceps and C. longiceps; the ca- 
rapace beilie fiarroweér and more convex than that of C. coniteps ; 
broader, and with a concavity arising from a more regular curvature 
than in C. breviéeps. The expanded parts of the ribs have an inter- 
mediate length with those of the two Chelones with which this spe- 
eimeén is eompared; and therefore is a difference independent of age. 
The distinetion of C. convecd is still more strikingly established in 
the plastron, which in its defective ossification more nearly resembles 
that of the existing species of Chelone. All the bones, especially 
the xiplisternals, are more convex on their outer surface than in other 
turtles, recent or fossil. ‘The internal rays of the hyosternals are 
divided into two groups; the lower consisting of two short and 
strong teeth projecting inwards, while the rést extend forwards along 
the inner side of the epistéinals. Thé sdine character may be ob- 
Setved in the corresponding processes of the hypostetnals, but the 
éxtérrial process is relatively much harrower than in C. breviceps. 
The following differences are stated to distinguish the sternum of 
C. convexa from that of C. Mydas. ‘The median margin of the hyo- 
sternals forms a gentle cutve, not ab angle: that of the hyosternals 
is likewise curved, bit with a slight notch. The longitudinal ridge 
on thé external surface, and near the median margin of the hyo- and 
hyposternals, is less marked in the Sheppey fossil ; especially in the 
hyposternals, which are characterized by a smooth ound in their - 
middle. 
The suturé between the hyo- and hyposternals is nearer to the 
external transverse radiated process of the hyposternals. The me- 
dian vacuity of the sternal apparatus is elliptical in the Chel. con- 
vera, but square in the Chel. Mydas. 
The characteristic lanceolate form of the epistérnal bone in the 
-séfitis Cheloné is well séen in the present fossil. 
