138 Prof. H. Gr. Seeley. The Shoulder Girdle 



ovate at first, but afterwards deeper. The centrum is always wider 

 than long, and has an oblique margin, which is absent in Gimolio- 

 saurus. The humerus and femur are deeper than wide proximally. 

 In the fore-arm there are three bones in a row, of which, ulna and 

 radius, like the tibia and fibula, are broader than long. There may 

 sometimes be a fourth bone in this row (C. Manselli, Hulke sp.). 

 The phalanges are not compressed. The types are from the Kimeridge 

 Clay, and include P. megadeirus and P. Manselli. 



These genera are distinguished by the extremities, though the 

 vertebral articulation of the zygapophyses and many parts of the 

 skeleton furnish differential characters. 



Both genera are defined from Polycotylus and Piptomerus by the 

 length of the dorsal centrum. The bevelled or rounded margin to the 

 articular face of the centrum separates them from Gimoliosaurus. The 

 absence of side-to-side compression of the centrum distinguishes them 

 from PJlasmosaurus. And they are separated from Trinacromerurn by the 

 structure of the glenoid cavity for the humerus, and the small number 

 of uncompressed phalanges in the digits. Hence, without disregard 

 of generic characters, and the facts of stratigraphical distribution, it 

 seems impossible to follow the British Museum Catalogue, which 

 enlarges the genus Gimoliosaurus to make it comprise all these 

 Elasmosauridee. And it will presently become evident that in Murceno- 

 saurus the diversity of modification found in the clavicular arch is 

 such as may define sub-genera within its present limits. 



Notwithstanding the diverse aspects of the shoulder girdle in the 

 Elasmosauridse and Plesiosauridas, and the circumstance that inter- 

 mediate types are at present unknown, the difference between them 

 is essentially in the fact that in all Elasmosaurians the supposed pre- 

 coracoid region is ossified so as to come into median union with the 

 •coracoid by suture, and co-ossified with the scapula so as apparently to 

 be an inseparable part of that bone ; and it is these precoracoid portions 

 of the scapulas which alone meet each other in the median line, as do 

 the precoracoid bones in Procolophon (' Phil. Trans.,' 1889, B, PI. 9, 

 fig. 9). In all Plesiosaurians, on the other hand, the precoracoid, if 

 developed, remains cartilaginous ; but I infer that a cartilage always 

 extended from the anterior margin of the coracoid to the anterior 

 extremity of the scapula, and by ossification of such cartilage the 

 Plesiosaurian shoulder girdle would become Elasmosaurian. 



§ 2. The Clavicular Arch in the Elasmosaurians discovered by 

 Mr. A. N. Leeds in the Oxford Glay. 



The clavicular bones may be placed anterior to the scapulo-precora- 

 coids, partly under them on their visceral surface, but they never 

 extend back to meet the coracoid bones,. as in Plesiosaurus. Or they 



