574 
angle thati in the C. breviceps, in which the vertebral scutes have 
the more regular hexagonal form of those of the C. Mydas. 
The plastron is more remarkable than that of the C. breviceps for 
thé extent of its ossification, the central cartilaginous space being 
reduced to an elliptical fissure. The four large middle pieces, called 
hyosternals and hyposternals, have their transverse extent relatively 
much seater, a8 compared with their antero-posterior extent, than 
iti C. breviceps. The median margins of the hyosternals are deve- 
loped in short toothed processes along their anterior two-thirds; and 
the médian margins of the hyposternals have the same structure 
along thei posterior halves. Rae 
The xiphisternals are relatively broader than in C. breviceps or in 
uny Of the existing spécies, atid aré united together by the whole of 
their median margins: The éntostérnal piece is flat on its under 
surface. Perna, * 
Each half of the plastron is mové regularly convex than in C. My- 
das. The breadth of the stérnum along the median suture, uniting 
‘the hyosternals and hyposternals, is five inches; and the breadth at 
‘the junction of thé xiphisternals with the hyposternals is two inches. 
Thé posterior paft of the cranitiii is preserved in this fossil, with- 
drawn beriéath the anterior part of the carapace ; the fracture shows 
the osseous shield covering the temporal fossee ; and the pterygoids 
temiin; exhibiting the wide aid deép groove which runs along their 
under part. 
It has been most satisfactory, the author says, to find that the two 
distinct species of the genus Chelone, first détermined by the skulls 
ofily; shotild thus have been éstablished by the subsequent observa- 
tioi of their bony cuifassés ; and that the specific differences mani- 
fested by thé cuirasses should be proved by good evidence to be cha- 
racteristi¢ of the two species founded on the skills, 
Thus the portion of the skull preserved with the carapace first 
deseribed; served to identify that fossil with the more perfect skull 
of the Chelone brevieeps, by which thé species was first indicated. 
And, again, the portion of the carapace adhering to the perfect skull 
of the Chelone longiceps equally served to connéct with it the nearly 
complete osseous buckler, which otherwise, from the very small frag- 
mefit of the skull remaining attached to it, could only have been 
assigned conjéctirally to the Chel. longiceps ; an approximation which 
Would have been the more hazardous, since the Chel. breviceps and 
Chel. longiceps are not the only turtles which swarm those aiicient 
seas that received the enormous argillacéous deposits of which the 
isle of Sheppey forms a part. 
3. Chelone latisculata.—A considerable portion of the boty cuirass 
of a yoting turtle from Sheppey, three inches in length, including 
the 2nd to the 7th vertebral plates, with the expanded parts of the 
first six pairs of ribs, and the hyosternal and hyposternal elements 
of the carapace, most resembles that of the Chelone coniceps in the 
form of the éarapace, and especially in the great transverse extent of 
the above-named parts of the sternum; it differs, however, from the 
Chel. longiceps and from all the other known Chelonites in the great 
