606 



ME. J. W. DAVIS ON CHONDEOSTEUS ACIPENSEEOIDES. 



present, and the ventrals are represented by a mass of bony ray 

 broken and fragmentary, but indicating clearly the position of the 

 fins. The anal fin is wanting in this specimen. The length of the 

 fish, from the tip of the upper lobe of the tail to the anterior extremity 

 of the head, so far as preserved, is 44 inches. Of this length, the tail 

 occupies 12 inches ; and the head, from the posterior margin of the 

 scapular arch to the anterior extremity of the mandible, is 9*2 inches. 

 The height of the posterior part of the head is 9 inches. The body 

 behind the head decreases rapidly in size ; between the dorsal sur- 

 face and the pectoral fin there is 7*5 inches. At the anterior base of 

 the dorsal fin, ten inches further back, the depth has decreased to 

 5'5 inches, and this is reduced at the tail to 2*5 inches. The ex- 

 pansion of the caudal fin is very rapid, the extremities of the lobes 

 being 13 inches apart. The single dorsal fin is separated from the 

 occiput by a distance of 11*5 inches; its base extends 6*5 inches, 

 and its posterior extremity is 5*5 inches from the base of the caudal 

 fin. The ventral fin is opposite to the posterior part of the dorsal ; 

 its anterior rays are separated from the anterior rays of the pectoral 

 fin by a distance of 13 inches, and from the base of the tail by 

 about 7 inches. Its base was between 4 and 5 inches in extent. 



The head in this specimen is proportionately larger and deeper 

 than the body of the fish. This comparative largeness is due, to 

 some extent, to the effects of lateral pressure during fossilization, 

 by which the skull has been squeezed flat on its side. The cranial 

 bones, which covered the surface of the crown, are pressed into a 

 lateral position, dividing down each side from the median suture. 

 The facial and maxillary, the hyoid and branchial elements are also 

 pushed considerably from their proper position, though they still 

 maintain their relative positions with regard to each other. 



The head, as preserved in this specimen, has an almost circular 

 outline, with a diameter of about 9*0 inches (see diagram, p. 607). 

 The anterior termination of the snout is absent, and no evidence is 

 afforded as to the length to which it may have extended forward, 

 this part of the specimen having been most unfortunately broken and 

 lost during its extraction from the matrix. 



The cranium was protected by dermal bones or scutes. The 

 upper surface was enveloped by large plates, which extended down- 

 wards so as to encircle the orbit, and the postero-lateral surfaces 

 were covered b}^ a large expansion of the opercular apparatus. The 

 anterior portion of the head beneath the orbit does not exhibit any 

 traces of external defence. In this respect the fossil Cliondrosteus 

 differs materially from the existing Sturgeons, in which the whole 

 of the superior surface of the head is enveloped in osseous plates. 

 The opercular apparatus is large and well developed ; its posterior 

 margin rests on the depressed anterior surface of the scapular arch. 

 The operculum (28 *) is 3*5 inches in height and 3*0 inches in 

 breadth ; its anterior margin is somewhat angular, and the inferior 

 one forms a concavity fitting to the upper convex margin of the 



* The numbers thus given in brackets are those employed by Sir Eichard 

 Owen to indicate the bones of the head. 



