Dr II. H. Traquair on Choiidrosteus acipenseroides. 359 



margin about the middle of its extent, and then diverges 

 downwards and backwards towards the angle. Sir Philip 

 Egerton describes the mandible of each side as a single bone, 

 and no doubt it is mostly composed of a large dentary 

 element {d., Fig. 3). But at its posterior extremity two other 

 elements are undoubtedly present, of which one, the upper, 

 may be reckoned as articular, while the other beneath it is 

 unquestionably the angular (ag.). 



No trace of teeth can be seen in connection with either 

 jaw; Chondrosteus in this respect, as in so many others, 

 resembling the recent Acipenser. 



Branchial skeleton. — Abundant remains of ossified ccrato- 

 hrancliials (Fig. 2, c.h) are seen in many of the specimens, 

 but we need not be detained at present by entering into 

 detail as to this part of the skeleton. 



. Shoulder-girdle. — Sir Philip Egerton states that whereas, 

 in Acipenser three bones are present in the shoulder-girdle, 

 viz., supra-scapular (post-temporal), scapular (supra- clavi- 

 cular), and coracoid (clavicle), in Chondrosteus the "scapula" 

 and "coracoid" have coalesced. In Acipenser there are, 

 however, at least four membrane bones of the shoulder- 

 girdle ; the post-temporal being, however, immovably arti- 

 culated with the cranial shield, while the fourth element is 

 the large infra-clavicular plate. And Mr Davis, as regards 

 Chmidrosteus, is undoubtedly right in maintaining that the 

 "scapula" and "coracoid" are not fused, though, as he 

 enumerated only three elements in the shoulder-girdle, he 

 seems not to have observed the infra-clavicular. 



The 'post-temporal (Figs. 1 and 2, p.t.) is a somewhat three- 

 cornered plate placed behind the posterior margin of the 

 cranial shield, and having its anterior margin overlapped by 

 the supra-temporal bones. This is followed by the supra- 

 clavicular (Fig. 2, s.cl), an oblong bone passing obliquely 

 downwards and backwards, and having its upper extremity 

 obliquely perforated by the side-canal. Its distinctness from 

 the clavicle is obvious in every well-preserved specimen. 

 This clavicle (Figs. 2 and 8, cl.) differs from that of the 

 Sturgeon in not being so much developed inferiorly, so that 

 the next element, the infra^clamcidar {i.cl.), articulates to its 



