78 PISCES. 



typical genera this ganoine extends over some of the anterior 

 rays of each of the fins. Osteolepis, Thursius, and Diplopterus 

 exhibit a pineal foramen in the frontal region (fig. 59), and are 

 characteristic of the Lower Old Red Sandstone of Scotland. 

 Megalichtliys has no pineal foramen, and ranges throughout the 

 Carboniferous and Lower Permian both in Europe and America. 



Sub-Order 3. Actinistia. 



Some of the Palaeozoic Crossopterygians became extremely 

 specialized chiefly by degeneration, forming the sub-order ACTI- 

 NISTIA with the single family of Ccelacanthidae. These have 

 perhaps the most remarkable range of all known extinct fishes, 

 occurring almost unchanged throughout the whole series of 

 formations from the Lower Carboniferous to the Upper Chalk. 

 The group is specialized in the large symmetrical caudal fin, 

 which exhibits a series of supports directly apposed to the 

 neural and ha?mal arches, equalling in number both these and 

 the overlapping dermal rays. It is also specialized in (i.) the 

 fusion of the bones of the pterygo-quadrate arcade, (ii.) the 

 reduction of the infradentaries to one, (iii.) the reduction of the 

 opercular apparatus to the operculum on each side and a pair 

 of gular plates, (iv.) the loss of the baseosts in the anterior 

 dorsal fin. and (v.) the ossification of the air-bladder. An un- 

 determined genus is represented by one fragmentary fossil 

 from the Upper Devonian of Germany; Ccelacanthus ranges 

 throughout the Carboniferous and Permian ; Undina (fig. 60) 

 is the best-known Jurassic genus ; while Macropoma is Upper 

 Cretaceous. 



Macropoma. As fine specimens of this fish occur almost uncrushed 

 in the English Chalk, it serves best to illustrate the osteological characters 

 of the family to which it belongs. The chondrocranium is extensively 

 ossified, but there is no interorbital septum. The membrane-bones of the 

 roof form a continuous shield, the short parietal region being nearly 

 parallel with the base of the skull, the longer frontal region meeting this 

 in an angle and sloping downwards. The hinder region comprises a pair 

 of large bones meeting in the middle line, evidently to be regarded as 

 parietals, flanked postero-externally by a pair of triangular bones, which 

 appear to represent the squamosal fused with the supratemporal. The 

 frontals are long and narrow, separated by a suture at the median line and 



