ShuuWer Girdle in Ichthyosauria and Sauropterygia. 



was 6' 7 cm. and the height 9 cm. I have not yet found conclusive 

 evidence of the coexistence of such a pair and of an azygos piece in 

 any of the Plesiosaurian remains in the British Museum, and I have 

 thought it probable that the azygos condition was a later stage 

 reached by the fusion of the paired elements as happens in the an- 

 alogous case of the Anourous omosternum. 



Mr. A. Leeds, however, assures me that he possesses one instance 

 of such association of two lateral and a median piece, which he had 

 placed for examination in Professor H. G. Seeley's hands. Such 

 association, if established, would not, however, prove the three pieces 

 to be not omosternalia but clavicular. 



Gotte has shown that in Eeptilia the interclavicle arises by the 

 coalescence of a piece segmented off from the mesial ends of each 

 clavicle. In Anoura, Parker has shown that the omosternum is formed 

 by the fusion of two pieces segmented off from the anterior extremi- 

 ties of the epicoracoids ; in this instance, if a remnant of each lateral 

 segment retained its distinctness, there would be a perfect accord in 

 the principle of the construction of the omosternal and clavicular 

 parts. 



By Professor H. G. Seeley the thinness and the surface texture of 

 those pieces is considered decisive of their being originated by the 

 ossification of membrane (not of cartilage), an origin which he re- 

 gards as decisive of their being clavicular and inter clavicular, and as 

 outweighing the anomaly of their " visceral position." It is possible 

 that they are membrane bones, but this is not yet absolutely certain ; 

 whilst their deep position, unknown, if clavicular pieces, in any other 

 Vertebrate skeleton, is not disputed. I submit, then, that the weight 

 of evidence is still in favour of an omosternal homology. 



Professor H. G. Seeley argues that these bones cannot be omoster- 

 nalia, because in every existing animal which has an omosternnm a 

 sternum also is present, but in no Sauropterygia is there even any 

 trace of a sternum. The usual association of omosternum and 

 sternum may be perfectly true, but since the genesis of these two 

 parts is perfectly distinct, one being a derivative of the epicoracoids, 

 the other a derivative of the costae, the presence of a sternum is not a 

 necessary antecedent of that of an omosternum. Moreover, though 

 no objective evidence of a sternum in Plesiosauridee has been pre- 

 served, the whole homology of the pectoral girdle, and the high 

 degree of development of the abdominal ribs makes the existence of 

 a cartilaginous sternum a very probable circumstance. Such sternum 

 might not imprint any trace of cartilaginous attachment or other 

 mark of articulation on the postero-internal parts of the coracoids, 

 since their articulation with it might be simply diarthrodial, much 

 as in Lacertilia, in which the border of the coracoid is simply received 

 in a corresponding groove in the sternum. 



