PURBECK SERIES. 
87 
scutum, the first neural plate is not divided. The im¬ 
pressions of the vertebral scutes pass over the first, third, 
fifth, ninth, and tenth neural plates, as in Pleurosternon 
emarginatum and P. Sedgwicki. The costal scutes are 
very narrow, the second being 4 inches long and 2 ^ 
inches wide, and the fourth scute is very small. It is 
readily distinguished from Pleurosternon emarginatum, 
the only species with which it could be confounded, by 
the absence of the emargination, the undivided first 
neural plate and the broad marginal plates. 
3 Pleurosternon Oweni. 
This carapace has lost the nuchal and most of the mar¬ 
ginal plates. Length from pygal plate to anterior margin 
of the first neural plate (which is undivided) 19 inches, 
breadth over the third costal plates 14^ inches; from 
which the perfect specimen may be presumed to have 
measured 22 inches by 17 inches, and to have had much 
of the form of Pleurosternon ovatum, being relatively 
almost as narrow. 
The carapace appears to have been more than usually 
convex, for, though it is now more convex than any 
other specimen, the costal plates have all parted from 
each other at their marginal borders from compression. 
The first vertebral scute is half an inch narrower than 
the second. 
It is readily distinguished from Pleurosternon ovatum 
by the marginal scutes extending far on to the costal 
plates, while in that species they are almost coincident 
with the marginal suture. The costal scutes are rela¬ 
tively narrow. The eighth neural plate is very small 
and quadrate, and only half the length of the ninth 
plate. 
4 Pleurosternon typocardium. 
Perfect carapace, with deep and wide nuchal emargi¬ 
nation and well-marked pygal emargination. Length 
along the neural plates 11| inches, breadth over the 
region of the fourth costal plates 11 \ inches. It differs 
from all other species in having the sutures between the 
