C. W. Andretvs — On a Young Plesiosanr. 243 



which do not unite below to close a hsemal canal and rarely fuse 

 with the centra. 



The abdominal ribs are strongly developed. They are arranged 

 in several transverse rows, each consisting of a median piece and 

 paii'ed lateral pieces. In a former paper ' I stated that there are two 

 of these lateral pairs, and in the present specimen they are so 

 mounted. In Mr. Leeds' collection, however, in a specimen^ in 

 which the ventral ribs are retained in their natural position through 

 being partly imbedded in a hard concretionary mass of clay, there 

 are clearly three lateral pairs, and this is also the case in, at least, 

 some of the Liassic Plesiosaurs. 



The pectoral-girdle has been figured and described in the paper 

 just quoted. 



The humerus at this stage shows scarcely any trace of the great 

 distal expansion which characterizes it in the adnlt, but the radius is 

 already very large compared to the other paddle-bones. 



The ilium is a curved rod of bone, laterally compressed at its 

 upper end and rounded at the lower, which in this specimen was 

 evidently covered in life with a cap of cartilage, the articular 

 surfaces not being ossified. In an adult ilium (see Figure) of the 

 same species the lower end bears two articular surfaces — the larger 



Pelvis of adult of Cryptoclidus Oxoniensis seen from within. 

 il, ilium ; j!?2«, pubis ; is, ischium ; acetab, acetabulum. 



for union with the ischium, the smaller forming a portion of the 

 hinder edge of the oval acetabulum ; there is no surface for the 

 pubis. The hatchet-shaped ischium has three subequal articular 

 surfaces at its upper end, the posterior for the ilium, the median 

 forming the middle and greater part of the acetabulum, and the 

 anterior for the pubis. This latter is a broad plate of bone with 

 convex anterior and concave posterior border ; proximally it unites 

 with the ischium and forms the anterior portion of the acetabulum. 



The femur is rather less expanded distally than the humerus, 

 a difference much more marked in the adult. The remainder of 

 the hind-paddle calls for no special notice. 



Plate IX. was prepai-ed from a photograph taken by A. Gepp, Esq., 

 of the British Museum (Natural History), to whom my cordial 

 thanks are due. 



1 Ann. Mag. Nat. Hist. ser. 6, vol. xv (1895), p. 334. 



* I am indebted to Mr. Leeds for drawing my attention to this specimen, 



