and Clavicular Arch in Sauropterygia. 



125 



fessor Huxley regarded it as interclavicle and clavicles (' Anatomy of 

 Yertebrated Animals,' 1874, p. 210). In 1874 I figured the clavicles 

 as posterior to the interclavicle in Plesiosaurus Hawkinsi, and drew 

 attention to the similar condition in PI. laticejps (' Geol. Soc. Quart, 

 Journ.,' 1874, p. 444, since figured by Zittel). Mr. Hulke, in 1883 

 (" Presidential Address, Geol. Soc," p. 20), regards these ossifications 

 as indivisible, and names the mass omosternum, thus reverting to the 

 hypothesis that the ossifications have a cartilaginous origin, and are 

 episternal. It follows from Mr. Hulke's views that the reputed 

 clavicles of Nothosaurus are precoracoid, and the median bone between 

 them is the omosternum. 



The late Professor W. K. Parker fully discussed the omosternum 

 in the Vertebrata. It is found in Mammals and in Anura, but is 

 not present in all Anura, and is not always ossified. In the genus 

 Calamites it appears to extend slightly on the visceral surface of 

 the precoracoids. In the Amphibian group which it characterises 

 clavicles are probably not found, so that it is in place of an inter- 

 clavicle, if it does not represent it. It is sometimes single, sometimes 

 paired, but never tripartite, as the median bone among Sauroptery- 

 gians. Among Mammals Mr. Parker found the omosternum (paired) 

 uniting with the sternum, while laterally it is continued by the 

 clavicles, though there is a pair of small cartilages, termed pre- 

 coracoids, between it and those elements of the skeleton. In the 

 Monotremata the interclavicle is in the position of the omosternum. 

 In Anguis fragilis Mr. W. K. Parker figures both interclavicle and 

 clavicles, but there is no omosternum. The omosternum behaves 

 as though it were the name given to the interclavicle when that 

 element ossifies from cartilage. 



A sternum is developed in every existing animal in which the 

 omosternum is present, but in no Sauropterygian is there ever any 

 trace of a sternum, so that there is nothing to suggest an omosternum. 

 The omosternum has not been recognised in any existing order of 

 Reptiles, and the Sauropterygia is the only fossil type except the 

 Nothosauria in which it has been supposed to be found. That sug- 

 gestion appears to rest upon the fact that the omosternum. is found 

 anterior to the precoracoids in certain existing Amphibia. There is 

 the circumstance that the bones in Plesiosaurus extend on the visceral 

 surfaces of the scapulae and coracoids, while the clavicles in Ichthyo- 

 saurus are on the anterior and ventral surfaces of the same bones ; bufc 

 no animal is known in which the omosternum is developed in the 

 position of the bone which has been so named in Plesiosaurus, and, so 

 far as position goes, there is no evidence known to me which suggests 

 that the bones in question should be omosternal rather than clavi- 

 cular. 



The omosternum has never been shown to consist of a "J"- or \/- 



