107 



the broad sternum and simple tail of the former genus ; and pos- 

 sesses, in common with the latter, a large head, and the peculiar 

 plates which are situated between the outer extremities of the pec- 

 toral and abdominal, and the marginal dorsal plates. It differs from 

 Chelydra, however, in the peculiar plate which covers the symphysis 

 of the sternum being here comparatively very small, not exceeding 

 in size the axillary and inguinal plates, and in its being inserted in 

 tli€ same line with them. 



The only species known was characterized as the 



Platysternon megacephalum. Plat, capite brunneo, obscure 



nigro radiato : tcstd supierne saturate brunnea, hifrh ■pallide jlava .- 



marginibus scutellorum sulcis aliquot ohscuris striisque radian- 



tibus confertis. 



Long, testae, 3^ unc. ; sterni, 24 : latitudo testse, 2|- ; sterni an- 



tice, 24-: long, capitis 24^3 caudae, 3. 

 Hab. in China. 



In illustration of the conterminous genus Emys, Mr. Gray exhi- 

 bited a specimen of the Em. Caspica, Schw., recently obtained from 

 the Mediterranean. 



Mr. Gray also exhibited a specimen of the animal (Ocytkoe) found 

 in the shells of the genus Argo7iauta, in illustration of some obser- 

 vations on the disputed question of its parasitic or non-parasitic 

 nature. He stated that he had lately examined ten specimens, four 

 of them referable to Ocythoe Crunchii, and the remainder to Ocu- 

 tlio'e antiquorum ; there being, however, little to distinguish them 

 except the size. All these specimens, as well as all those which 

 have been figured, were females, and had eggs inclosed in the hin- 

 der part of the shell, in the cavity which is uniformly found behind 

 the body of the animal. In all, the posterior sip/io?t was placed more 

 or less exactly in the keel of the shell, but the body did not always 

 occupy a s)'mmetrical position with regard to it, the eye of one side 

 being sometimes nearer to the spire than that of the opposite side. 

 Only one or two of these individuals had their bodies n)arked with 

 the ridges of the shells, the impressions of which were, however, 

 mostly observable upon the arms. The animals all appeared to be 

 retained in the shells by the inflection of the anterior pair of arms. 

 Mr. Gray added that he had also lately seen several specimens pre- 

 served without shells, and having their bodies shaped exactly like 

 that of the common Octopus, without the slightest appearance of 

 their having been inclosed in shells : the history of these specimens 

 he was unable to trace, and he could not therefore aflirm that they 

 were found in the state in which he observed them. 



From these facts Mr. Gray stated that he was inclined to regard 

 it as probable that the Ocythoe is only parasitic in the shell of Ar- 

 gonauta ; that the shells are only resorted to by females during the 

 breeding season for the protection of their eggs ; and that the chief 

 purpose of the dilated portion of the anterior arms is to retain the 



