>30 



VERTEBRATE SKELETON 



developed in some fishes and tend towards reduction or absolute loss 

 in some Tetrapoda. In some Ganoids (sturgeon, etc., fig. 243) these 

 bones are clearly dermal — parts of the skin — and as their homologues 

 are evident in other forms, it is clear that they are phylogenetically 

 derived from dermal ossifications, no matter how deep they may lie. 

 Most prominent of these bones and persisting longest is the clavicula^ 

 which overhes more or less of the anterior ventral part of the coracoid 

 cartilage and above this is the cleithrum which may extend dorsally 

 over the scapular cartilage. The clavicles of the two sides may meet 



in the middle fine, or sternum or 

 episternum (membrane) may inter- 

 vene. There may be a postclei- 

 thrum posterior to the cleithrum, 

 while the series of membrane bone 

 may be continued dorsally by one 

 or two supracleithra, the more 

 dorsal of these (supracleithrum or 

 posttemporal) being intimately 

 connected with the posterior part 

 of the skull, anchoring the girdle 

 more firmly. Outside of Tele- 

 ostomes (and possibly some Thero- 

 morphs — Cotylosaurs are said to 

 have a cleithrum), none of these 

 membrane bones, clavicle and 

 episternum excepted, are known. 

 Usually three cartilage bones develop in either half of the 

 pelvic girdle (fig. 242), an ilium dorsal to the acetabulum, ventral to 



Fig. 243. — Posterior and lateral views 

 of left half of shoulder girdle of Acipenser 

 sturio (Biitschli, 'lo). Permanent carti- 

 lage finely stippled; cartilage bones, 

 coarse stippling; membrane bones white. 

 cl, clavicle; cm, cleithrum; corac, cor- 

 acoid; g, glenoid area; A'', nerve through 

 coracoid foramen; pc, postcleithra; 5*', 

 supracleithra; 55, suprascapula. 



anterior of, these being universally called precoracoid (or procoracoid) , while the name 

 epicoracoid is applied to a cartilage (or bone) uniting the medial ends of these two. 



Adherence to the 'law of priority' would necessitate a change of names in the lower 

 groups, but the priority fetich should not be allowed to cause greater confusion than now 

 exists, when by setting it aside, greater simplicity may be had. This would involve 

 only a change of epicoracoid to precoracoid in the Monotreme. The fact that the 

 human coracoid process is a double structure does not necessitate the term metacoracoid 

 for the posterior element of the lower orders. 



1 Earlier writers identified the more dorsal of the piscine bones overlying the girdle 

 as the homologue of the human clavicle. Restoring this name to the proper bone 

 (infraclavicle of the earlier terminology) the bone formerly called clavicle is the clei- 

 thrum, while in the same way the supraclavicula become the supracleithra, the post- 

 temporal being the upper supracleithrum and the postclavicula the postcleithrum. 



