1873.] SEELEY CETABTHROSATJRTJS WALKERI. 507 



narrow oblong at the distal end. The proximal end is 1-j^- inch 

 from side to side in the direction of the extension of the distal 

 end, and 1^ inch in the direction at right angles, thns differing 

 essentially from Ichthyosaurus ; but, as in that genus, the superior 

 outline of the head is somewhat flattened, while the inferior outline 

 is very convex ; the head is hemispherically rounded, and so much 

 of the original surface as remains is pitted over with terminal blood- 

 vessels smaller than those of Plesiosaurus, and shows indications 

 of the bone having been there sheathed with a thick articular car- 

 tilage. The distal end, 1-| inch long and -| inch wide, is rather 

 wider towards the tibial side, and rather narrower towards the 

 fibula. The distal end shows three articular surfaces — two concave 

 and subquadrate, and a small lunate articulation on its fibular 

 margin. Between the head and the distal end the sides of the bone 

 are concave, rather more so on the fibular than on the ulnar side. On 

 each side of the bone, at right angles to the distal end, is given off an 

 enormously elongated compressed troehanteroid process, which, even 

 as preserved, widen the bone to 1^ inch, while the constricted shaft 

 at right angles measures -^ inch only. On the underside the bone is 

 gently convex from one trochanter to the other ; on the superior 

 face a longitudinal convex ridge extends from the distal end towards 

 the head, while the parts on each side of it are made concave by the 

 singular compression of the troehanteroid processes. From the con- 

 dition of preservation, some difficulty may be felt in deciding how 

 far these processes extended towards the proximal articulation ; but 

 as on both sides they are seen to run nearly to the head of the bone, 

 they were probably there given off. The processes may have been 

 of equal lengths, and have measured nearly 3 inches from side to 

 side when perfect, making the bone wider than long. 



Some Ichthyosaurians from the Lias in the British Museum show 

 a small troehanteroid process towards the distal end of the bone on 

 the fibular margin, quite distinct from these. 



On the subovate form of the head of the bone, and on the deve- 

 lopment of the lateral trochanters, I base the genus Getarthrosaurus. 

 It may be considered to present a resemblance one degree nearer to 

 the femur of the monotremes than that of Ichthyosaurus, a resem- 

 blance having its chief interest in other parts of the skeleton. 



The name is not intended to indicate cetacean affinities, but 

 merely the general resemblance of the bone to ■ the humerus of a 

 porpoise, which is sufficiently marked to arrest attention. 



Discussion. • 



Sir P. Egerton was inclined to regard the trochanters as vertical 

 rather than as lateral. 



Mr. Seeley remarked that in calling the two trochanters lateral, 

 he was guided by the position in which it appeared to him that the 

 limbs were carried during the life of the animal. 



