324 J. W. DAVIS ON THE GENUS PLEUKACANTHUS, AGASS. 



•wards, which may have been connected with the nostrils. Dr. Kner 

 considers that there were certainly four or five gill-arches, which 

 were furnished vtdth a few long teeth. The gill-arches were at- 

 tached to the substance of the hyoid bones, their union with the 

 shoulder-girdle being similar to that in the Squalida?. Immediately 

 behind the large jaws the head seems to have been contracted in 

 width. From the centre of this part the spine emerged. It was 

 not connected by any articulation, but appears to have been simply 

 implanted in the cartilaginous mass of the occipital region. In all 

 the specimens where the spine is present, it is found lying in close 

 proximity to and along the dorsal surface of the fish. 



In the vertebral column the vertebrae were cartilaginous, except 

 the apophyses to which the ribs were attached; these appear to 

 have been more or less bony. The ribs were short and somewhat 

 rudimentary, broad at the base for articulation, and ending in a 

 point. Immediately behind the spine there originated a dorsal fin, 

 which extended along the back to the caudal extremity. Besides 

 the spinous processes attached to the vertebrae and the spinous rays 

 which constituted the fin, there were two series of interspinous^ 

 bones, the latter articulated in the usual manner. 



This arrangement probably extended a short distance beyond the 

 abdominal fin. The interspinous bone next to the spinous process 

 of the vertebrae then disappears, the second one being continued 

 nearly to the caudal extremity of tlie body. A fin also extended 

 along the ventral surface of the body, and, joining the dorsal one, 

 formed a single-lobed tail. 



The pectoral arch was large and strong. Dr. Goldfuss* describes 

 it as being built up on either side of an inner bone composed of a 

 single piece, which, on the hinder part, is bent on its outer edge in 

 the shape of a knee. This edge is beset with fin-rays. The ante- 

 rior ones are very short and thin ; those following are long and thick. 

 Just before the knee-shaped angle springs a strong, distinctly jointed 

 ray. It carries on its outward side seventeen thick strong fin-rays, 

 which become fibrous towards the end, and on its inner side a 

 number of smaller and closer fin-rays. It is not clear by what 

 means the knee-shaped bones were attached to the remaining bones 

 of the shoulder-girdle. In a well-preserved specimen of the pectoral 

 fin from the Cannel Coal at Tingley, all the larger fin-rays are 

 iointed, the joints between each articulation averaging about half 

 an inch in length. The fin has probably belonged to a much larger 

 fish than the one described by Dr. Goldfuss. AU the fin-rays were 

 semicartilaginous, with innumerable minute centres of ossification. 



The pelvic arch was constructed on a similar basis to the pectoral 

 one. A broad, short, knee-shaped bone sprang from the vertebral 

 column ; and to this was attached an articulated primary ray, as in 

 the pectoral fin ; but whereas in the latter small rays sprang from 

 the inner side of the primary ray, in the ventral fin only the outer 

 portion supported fin-rays. Dr. Kner describes the pelvic bones as 

 forming a ventral shield studded with hook-like appendages. These 



* 0_p. cit. 



