4 FOSSIL REPTILES OF DORSET. 



and swim in the water, had a much depressed carapace, the feet 

 were webbed only at the base, which enabled them to climb the 

 muddy banks of the rivers they frequented ; the rib-plate extends 

 over the whole length of the rib, and is articulated to the marginal 

 plates. The carapace and plastron are united at their borders. 



Two genera of this Order, Pleurosternon and Tetrosternon, occur 

 in the Middle Purbeck beds of Swanage. 



GENUS PLEUROSTERNON, Owen. 



This genus, as distinguished by Sir Richard Owen, K.C.B., 

 F.G.S., has an additional pair of elements mesosternals to 

 the typical number which are comprised in the under shell or 

 plastron, and united to the carapace by the marginal plates. The 

 four known species of Pleurosternon are limited to the Feather- 

 'bed horizon of the Middle Purbecks, Swanage. PI. continuum, 

 PL emarginatum, PI. ovatum, PI. laticostatum. 



PLEUROSTERNON CONCINNUM, Owen. 



The carapace of this species is oval, broadest anteriorly, 

 very flat, with scarcely any longitudinal elevation along the 

 centre or any other part. The plastron is long, flat, narrow, 

 and oval, the posterior emarginate. The length of the cara- 

 pace from the first nuchal plate to the posterior pygcd is 

 thirteen inches, the breadth across the third costal plate is 

 eleven inches, the length of the plastron thirteen inches, and 

 breadth six inches six lines. The forms and proportions of the 

 horny scutes that covered the plastron are shown by the narrow, 

 well defined impressions of their boundary lines. 



The carapace consists of a longitudinal series of ten median 

 plates, on each side of which, and united by sutures, is a series of 

 large lateral plates, and a series of marginal plates similarly united. 

 On the ventral surface are nine bony plates, which form a 

 continuous shield called the plastron. 



PLEUROSTERNON EMARGINATUM, Owen. 



This species differs from the preceding chiefly in the contour of 

 the borders of the plastron, the anterior being straighter, the 



