44 



Field Columbian Museum — Geology, Vol. II. 



in a median symphysis for a short distance back of this interclavic- 

 ular foramen. The under or convex surface appears only for a small 

 space between the ends of the scapulae, epicoracoid processes and the 

 episternum. Its outer extremity extends into a slender process, 

 broken away in the specimen figured, which reaches along the front 

 margin of the ventral ramus of the scapula to the base of the dorsal 

 ramus. (See PI. XIII.) 



The clavicle seems to be the most variable bone in the skeleton ; 

 I know of no form in which it is better developed. In Cryftoclidus it is 

 a small, triangular bone, meeting its mate in the middle line, according 

 to Andrews, the episternum being absent. In Plesiosaurus mudgei 

 Cragin (see PI. XXVII), the clavicle appears to have been smaller 

 and more triangular than in the present species. It is generally 

 assumed that this bone is the real clavicle by Seeley, Andrews, Fiir- 

 bringer and others, and one may, with Andrews, explain its position 

 in relation to the ventral ramus of the scapula by the peculiar method 

 of ossification of that bone. But, can its visceral relations to the 

 coracoid and episternum be explained as easily? It is true that many 



authors call the median, un- 

 paired piece the interclavi- 

 cle, a membrane bone, and, 

 if this origin is accepted, its 

 position in relation to the 

 clavicle is not remarkable. 

 But I am inclined, with Geg- 

 enbaur and Furbringer, to 

 believe that this central piece 

 is really the episternum. a 

 cartilage bone. Andrews 

 described the clavicle in some* 

 forms as showing a sutural 

 tendency with the scapula, 

 and in some forms, as Elas- 

 i)i o$a urns, it seems to become 

 entirely fused with that bone, 

 or if not, has disappeared 

 entirely. 



The episternum (inter- 

 clavicle) is a small, symmet- 

 rical bone deeply emargi- 

 nate in front and behind. The thin squamous margin on each 

 side underlaps the clavicle. These squamous borders seem to have 



Fig. 9. 

 Episternum of Trinacromerum ainmymum Will. 

 one-half natural size. 



