ORDER CHELONIA. IO93 



Much confusion has arisen in regard to this genus owing to a 

 plastron having been described under the name of Platemys Bullocki, 

 under the erroneous impression that it had been obtained from the 

 London Clay. Remains of Plearostemum are extraordinarily abun- 

 dant in the Purbeck of Dorsetshire ; and include specimens of all 

 ages, from the newly hatched young, with a carapace of a couple of 

 inches in length, to adult specimens which are close upon twenty 

 inches. Curiously enough, however, no specimen of the skull 

 seems to have been obtained. The young appears to have differed 

 considerably in the details of the shell from the adult ; thus, not 

 only was the pubis, as already mentioned, entirely unconnected 

 with the plastron, but the marginal bones encroached in a remark- 

 able manner upon the front of the nuchal. Moreover, it seems 

 that in very young individuals the vertebral shields were divided in 

 the middle. 



In Helochelys, of the Lower Greensand of Bavaria, we have an- 

 other genus also furnished with complete mesoplastrals, but ap- 

 parently without any articulation between the pubis and the 

 plastron. The shell is ornamented with a pustular sculpture re- 

 sembling that found in the genus Tretosternum, mentioned below 

 among the Chelydridce. The plastron differs from that of Pleuro- 

 sternum in that the xiphiplastrals were not notched ; and there 

 appears to have been a nuchal shield. 



The next two genera, constituting the family Ba'enidcz of Professor 

 Cope, may be at least provisionally placed here. Both are devoid 

 of a bony attachment between the pelvis and plastron. The genus 

 Platychelys (He/emys), typically from the Lower Kimeridgian litho- 

 graphic limestone of the Continent, is readily distinguished by the 

 number of irregular ridges and prominences on the carapace (fig. 

 1013), and by the width and irregular contour of the neural bones. 

 The mesoplastrals are small, and widely separated in the middle 

 line ; and the intergular shield is single. 



The genus Paena, from the Eocene of the United States, has the 

 mesoplastrals meeting only by a point in the middle ; and is further 

 remarkable for the presence of double intergular shields, and the 

 presence of a small additional costal shield in advance of the normal 

 first costal. The caudal vertebrae are opisthoccelous. Professor 

 Cope, who places this genus in the Cryptodira, regards it as indi- 

 cating a generalised type, showing marked signs of affinity with the 

 Pleurodira, and exhibiting traces of an imperfect connection between 

 the pelvis and the plastron. The extinct North American genus 

 Polythorax should perhaps be also placed in this family, although it 

 is not certain that it is not allied to the Cryptodiran Adocus. 



Here also may be mentioned the very imperfectly known genus 

 ArchczochelyS) of the English Wealden, in which, as observed above, 



