75 



humerus, I am unable to determine, owing to their close resemblance. The 

 vertebrae do not differ from those of the specimen just described. The limb- 

 bones are stout and expanded, and thinned distally ; this thinning is remark- 

 able, and indicates a much flattened metapodial region. The head is slightly 

 expanded, the articular face being turned obliquely to the inner face of the 

 shaft ; the surface is pitted for attachment of the articular cartilage ; two- 

 fifths the length from the proximal end is an extensive and exceedingly rugose 

 surface, as wide as the shaft, for the insertion of the adductor-muscles. 



Measurements. 



M. 



Diameter of the centrum of the lumbar vertebra 0.08 



Length of the (?) humerus 0.45 



Diameter of the head 0. 1-5 



Diameter of the shaft 0. 09S 



Diameter of the distal end (transverse), restored in part 0. 18 



Should the humerus have been related to the fore-limb, as in Plesio- 

 saurus dilichodirus, Conyb., the latter would have had a length of 4 feet 

 3 inches ; as the proportions of the radius and phalanges are shorter, the 

 limb was probably relatively shorter. If related to the total length, as in the 

 same Plesiosaur the humerus would indicate a length of 17tjr feet. The cer- 

 vical vertebrae become attenuated, as compared with the dorsals, to a greater 

 degree in Polycotylus than in Plesiosaurus. 



ELASMOSAUKUS, Cope. 



This genus has been more completely preserved to us than any other 

 American representative of the order. In the interpretation, however, con- 

 siderable care is necessary, as the form appears, at first sight , to reverse, to 

 a remarkable degree, the usual proportions of known reptiles. The scapular 

 arch, in the absence of the episternum, presents the same number of ele- 

 ments as the pelvic, and is not without resemblance to the latter, as it 

 exists in some species of the order. The fortunate preservation of the series 

 of cervical vertebras shows this to have been, in the typical species, three 

 times the length of the body; much exceeding in this disproportion that 

 known to exist in other species of the order. 



The neural arches are everywhere continuous with the centra, without 

 sign of suture, and are externally plane. The neural canal is exceedingly 

 small for the size of the. vertebra;, especially on the lumbar and caudal verte- 

 brae. 



