38 PISCES. 



suture. In Parexus (fig. 31) the broad, curved pectoral spine 

 (s) is supported by a hollow triangular element (b), with its 

 base directly apposed to the obliquely truncated end of the 

 spine and its apex produced into a short slender rod. Between 

 the two spines there is a pair of large and thin, triangular or 

 sickle-shaped elements (cl.) meeting in the middle line ; and the 

 surrounding squamation is arranged in direct relation to these 

 plates in such a manner that they are proved to be quite 

 superficial. The pelvic fins are always present, often as large 

 as the pectorals in the less specialized forms, smallest in the 

 latest forms. The squamation is very fine, closely covering the 

 whole of the trunk, and not enlarged even on the ridge of the 

 upper caudal lobe. The scales are rectangular or slightly 

 rhomboidal, with enamelled and frequently ornamented ex- 

 ternal surface ; and there is a single " lateral line " on the flank, 

 not piercing the scales but passing between two series. The 

 fin-membranes are also at least in part stiffened with quadrate 

 dermal granules of the same nature as those covering the 

 trunk, and these are often arranged in regular lines simulating 

 rays. 



Two families of Acanthodian fishes are known, one charac- 

 terized by the presence of two dorsal fins, the other by a single 

 dorsal fin. The former, or Diplacanthidae, are exclusively Upper 

 Silurian and Lower Devonian, while the latter, or Acanthodidae, 

 range from the Lower Devonian to the Lower Permian inclu- 

 sive. All the known forms are of relatively small size, the 

 majority being less than 0'3 m. in length. 



The Diplacanthidae are remarkable for the great develop- 

 ment of the dermal spines, there being sometimes not less than 

 four or five pairs between the pectoral and pelvic fins. In Gli- 

 matius (fig. 28) the fin-spines are remarkably broad and marked 

 with coarse longitudinal ridges, sometimes with posterior denti- 

 cles ; the first dorsal spine is not excessively elongated. Parexus 

 is nearly similar, but the first dorsal spine is relatively much 

 elongated and the fin-membrane does not extend so far as its 

 apex. These genera and Diplacanthus do not exhibit any 

 teeth ; but Ischnacanthus has a formidable dental armature. 

 In this fish there are widely-spaced, large conical teeth, inter- 



