665 



shoulder girdle consists only of the cartilaginous portion (scapula-coracoid); 

 the region of the shoulder girdle is covered like the rest of the body 

 by placoid scales. "When dermal ossifications appear on the skull we 

 also find such ossifications on the cartilaginous shoulder girdle. In 

 the Sturgeons (Chondroganoidei) for instance we find two such ossi- 

 fications covering the cartilaginous scapula-coracoid of both sides. 



The lower ossification; the larger one was formerly called the 

 infracla viele, the upper smaller one the clavicle, by Gegenbaur. These 

 two ossifications are also found in the Crossopterygians (Polypterus 

 and Calamoichthys) and the Dipnoi. By a comparison of these forms 

 with the Stegocephali Gegenbaur reached the conclusion, that the 

 lower element (infraclavicle) of the Crossopterygii and Dipnoi is 

 homologous to the clavicle of the Stegocephali ; the upper piece (cla- 

 vicle) to the dermal bone in the Stegocephali, named scapula, prae- 

 coracoid or clavicula. For this element he introduces the name 

 Clei thrum. The true clavicle of the fishes mentioned is the clavicle, 

 the bone formerly called clavicle is the cleithrum of the Stegocephali. 

 In the Rhomboganoidei (Lepidostei), Cycloganoidei (Amia) and the 

 Teleostei the cleithrum alone is present, in the Amniota the Cleithrum 

 is gone, but in the Stegocephalia, the Chondrostei, Dipnoi and Crosso- 

 pterygii we have both clavicle and cleithrum. 



We shall now consider the conditions in the Stegocephali more 

 fully. We always find one median and a pair of lateral dermal ele- 

 ments on the lower side of the body behind the hyoid region. These 

 were originally described as thoracic plates. They are homologous 

 to the epiplastra and endoplastron of the Testudines. There is quite 

 a considerable diversity in the form of these elements, as can be best 

 seen from the figures (Fig. 4 — 8). The cleithrum (scapula praecoracoid, 

 clavicle) is connected with the distal ends of the clavicles, and is 



Pig. 4. 



^(^ a 



Fig. 4. Shoulder girdle of Pelosaiirus laticeps, Credner. Restored from 

 Credner. 



Fig. 5. Shoulder girdle of Branchiosaurus amblystomus, Cred, Restored from 

 Credner. 



