256 Dr. It. H. Traquair — On Chondrosteus acipenseroides. 



approaclies close to the superior 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 ; 

 Cliondrosteus in this respect, as in so many others, resembling the 

 recent Acipenser. 



Branchial sheleton. — Abundant remains of ossified cerato-hrancMals 

 (Fig. 2. c. b.) 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. 



Shoidder-girdle. — Sir Philip Egerton states that whereas in 

 Acipenser three bones are present in the shoulder-girdle, viz. supra- 

 scapular (post-temporal), scapular (supra-clavicular), 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* 

 immoveably articulated with the cranial shield, while the fourth 

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

 Chondrosteus, 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- clavictdar (Fig. 2. s. cL), an oblong 

 bone passing obliquely downwards and backwards, and having its 

 upper extremity obliquely perforated by the side-canal. Its distinct- 

 ness from the clavicle is obvious in every well-preserved specimen. 

 This clavicle (Figs. 2 and 3, cl.) differs from that of the Sturgeon in 

 not being so much developed interiorly, so that the next element, the 

 infra-clavicular (i. cl.) articulates to its lower margin, extends higher 

 up, and comes to be opposite a considerable portion of the origin of 

 the pectoral fin. There is still another plate (Fig. 3, p. cl.), though 

 it is a very small one, appended to the posterior margin of the 

 clavicle, and this we may recognize as corresponding to the post- 

 clavicular of Polyodon and the Paleeoniscidge. 



I have seen no evidence of ossification in the scapulo-coracoid 

 cartilage, nor does any specimen afford a view of the base of the 

 pectoral fin, though in some a few dislocated radials m^y be seen in 

 this position. It is probable that the arrangements here resembled 

 those in Acipenser, and I own I am somewhat at a loss to understand 

 the " two bones " to which Mr. Davis alludes as " apparently repre- 

 senting the radius and ulna of Owen (coracoid and scapula of 

 Parker)." 



