24 FISH GALLERY. 



placoids of the skin are flatten ed and closely fitted together. The 

 calcifications of the cranium, jaws, and pectoral girdle present the 

 appearance of membrane bones, although bone cells are wanting. 

 The fins, both paired and median (except the caudal), bear each a 

 stout spine along the front edge, and in some cases there occur 

 between the pectoral and pelvic fin of each side a row of four or five 

 spines of a similar nature (see 11, and fig. 11). The cartilaginous 

 supports of the fins must have been insignificant, and the fin- 

 membranes but feebly supported by dermal fin-rays. The earlier 

 forms, those of the Upper Silurian and Lower Devonian, are 

 included in the family Diplacanthidse (e. g. Diplacanthus and Cli- 

 mat'ius) ; these have two dorsal fins. The family Acanthodidse (e. g. 

 Acanthodes) includes the later forms, those ranging from the Lower 

 Devonian to the Lower Permian, which have but a single dorsal 

 fin, and pelvic fins of smaller size than the pectorals. In Cli- 

 matius (11) the fin-spines are remarkably broad, and are marked 

 with coarse longitudinal ridges. The most perfect specimens 

 known are about seven inches in length, but fragments have been 

 found of specimens which must have measured eighteen inches or 

 more when complete. 



SELACHII (Modern Sharks and Rays). 



The order Selachii includes all the modern Sharks and Rays, 

 and a number of extinct forms as well. The great majority of the 

 Selachii are marine, but a few species live in fresh water. The 

 denticles of the skin are closely set to form shagreen, but in the 

 Rays there is a tendency for the spines to become fewer and 

 larger. The calcifications on the surface of the cartilages do not 

 resemble membrane bones, and have not the microscopic structure 

 of true bone. The basal cartilages of the pectoral fin, namely, 

 those in contact with the pectoral girdle, are usually three in 

 number, less commonly two ; there is no long segmented axis to 

 the fin. With a few exceptions the vertebral centra are well 

 calcified, but the form of the calcified layers varies considerably 

 (compare specimens 20, 28, 65, 80, 85, 90, 113 in Wall-cases 1-4). 

 The Selachii range back to Carboniferous and Permian times. The 

 suborders recognised are : — Notidani, Squali, and Raii. 



