126 Prof. H. Gr. Seeley, The Shoulder Girdle 



shaped median piece flanked by separate lateral ossifications as in 

 Plesiosaurus, while this condition parallels the interclavicle and 

 clavicles in all animals in which they are found. 



It has never been shown thatfany one of the bones in question in 

 Plesiosaurus retains a surface which has the aspect of having been 

 cartilaginous. On the contrary, every specimen which I have ex- 

 amined is more or less thin and squamous, with contours completely 

 ossified to sharp edges, even in the most immature specimens ; while the 

 interclavicle, when preserved, unites with the clavicles either by a thin 

 squamous overlap or by sagittal sutures. This condition seems to me 

 to demonstrate that the bones are membrane bones. I submit it 

 follows that they are clavicles, and therefore that the visceral position 

 of the clavicular arch, although anomalous, is not inconsistent with 

 clavicular homology. Bone for bone, the three clavicles in Plesio- 

 saurus seem to me to correspond to those of Ichthyosaurus and Notho- 

 saurus. In the former their union is usually squamous, in the latter 

 it is sutural. In Sauropterygia both conditions are found. The 

 proposal made to identify the three anterior bones in the shoulder 

 girdle in Nothosaurus as omosternum and precoracoids introduces the 

 precoracoid as a distinct bone,* which is not known to be paralleled 

 in any allied group of animals except the Anodomontia, in which 

 there is no omosternum, and where the precoracoids are differently 

 conditioned, being in the closest union with the coracoids, with a 

 well-developed clavicular arch. But when the supposed precoracoids 

 of Nothosaurus are recognised as clavicles, which rest by squamous 

 overlap on the visceral surfaces of the scapulae, like the clavicles of 

 Plesiosaurus, the clavicular arch is in harmony with that of the 

 Sauropterygia, and the supposed differences in its composition dis- 

 appear. 



There are two family types in the Sauropterygia defined by differ- 

 ences in the shoulder girdle and other characters, known as Plesio- 

 sauridae and Elasmosauridse, though the organic differences which 

 characterise them have not been fully set forth. 



II. Further Evidence of the Nature op the Clavicular Arch 



IN THE PLESIOSAURID^. 



§ 1. Nature and Limits of the Family. 



There are four principal genera of Plesiosauridas, which are named 

 Plesiosaurus, Eretmosaurus, Phomaleosaurus, and Pliosaurus. The 

 family is characterised by the cervical ribs being attached to the 

 vertebrae by more or less completely-defined double facets and by the 

 scapulas being separated in the median line by the clavicular arch, by 



* Hulke, loc. cit. 



