610 MR. J. W. DAVIS ON CHONDROSTECS ACIPENSEROIDES . 



and the inner extent of the bone cannot be distinguished. Between 

 the coracoid and the pectoral fin there are two bones connecting 

 them together. The anterior one, 0*7 inch in breadth and 0*25 in 

 length, is rounded in front, where it affords a substantial support to 

 the large anterior ray of the pectoral fin ; behind it is somewhat 

 attenuated. The second intermediate bone extends from the pos- 

 terior extremity of the one described to the termination of the base 

 of the fin, and is nearly 2*0 inches in extent ; to it are attached 

 the bases of the remaining rays of the pectoral fin. The two bones 

 probably represent the radius and ulna of Owen (coracoid and 

 scapula of Parker and Huxley). Sir Philip Egerton*, in his de- 

 scription of the genus Chondrosteus, states that the elements of the 

 bony girdle encircling the thoracic region, which in recent Sturgeons 

 are three in number, in the fossil genus are reduced to two, by the 

 coalescence of the scapula and the coracoid. " The external portion 

 [of the combined scapulo-coracoid] is crescentic, the concavity being 

 anterior for the reception of the opercular flap. At the point of 

 attachment of the pectoral fin the bone is thick and rounded ; the 

 upper or scapular portion expands gradually into a triangular 

 plate, thinning off towards the upper extremity for adjustment with 

 the suprascapular bone ; the lower or coracoid portion also expands 

 as it descends, sweeping under the thorax, and meeting the corre- 

 sponding bone of the opposite side on the median line. . . . Anteriorly, 

 each scapulo-coracoid expands into a broad concave plate, directed 

 inwards and forming the platform of the branchial cavity, and 

 partitioning it off from the thoracic cavity." In the specimen now 

 described the great lateral and anterior expansions do not appear 

 to be developed as in the specimens described by Sir P. Egerton ; 

 and the scapula and coracoid are separated, the two overlapping 

 midway between the attachment of the pectoral fin and the junction 

 with the suprascapular. The small bones connecting the scapular 

 arch with the pectoral fin in the species already referred to are said 

 to be a series of strong metacarpal bones, of which those supporting 

 the hinder rays are longest. A reference to the plate (op. cit. lxix. 

 57) on which these bones are represented will show that the meta- 

 carpals are not similar to the bones existing in this specimen, de- 

 scribed as equivalent to the ulna and radius ; they have the appear- 

 ance of being merely fragments of the rays of the pectoral fin which 

 have become detached. This pectoral fin is large and comprises 42 

 rays. The anterior ray is spinous, thick, with expanded base for 

 attachment ; it gradually tapers to a point, terminating 2-5 inches 

 from the base. The succeeding rays are the longest. In this specimen 

 there is preserved a length of 4-25 inches, but the distal extremities 

 are imperfect, and at least one inch more would be required to com- 

 plete the length of the fin-rays. The base of the fin nearly 

 approaches 3*0 inches in length, and the fin-rays gradually diminish 

 backward ; the posterior rays are not more than half the length of 

 the anterior ones. The rays increase, towards the distal end, by 



* " On Chondrosteus, an extinct Genus of the Sturionidaj found in the Lias 

 formation at Lyme-Begis," Phil. Trans, vol. cxlviii. 1858, p. 871. 



