2SO 



0STE0LEP1D0TI 



comparatively small ova. (The nostrils usually are dorsal ; but this 

 was certainly not always the case in the extinct Osteolepidoti ; see 

 below.) 



Division 1. 



Order OSTEOLEPIDOTI (Crossopterygii, Osteolepida). 



The Osteolepidoti flourished from the Middle Devonian to the 

 end of the Carboniferous epoch ; only one genus is known to extend 

 into the Permian, MegaUchthys, the largest of this order. Thanks 

 to the researches of Pander [312], Huxley [228], Traquair [443, 

 445, 450]), AVhiteaves [488], A. S. Woodward [505], and others, 



our knowledge of the ex- 



<M>1 



ternal form and internal 

 skeleton of the Osteolepidoti 

 is fairly complete. ' 



The scales vary in shape 

 from the rhomboid (Osteo- 

 lejris, Fig. 250) to the cycloid 

 form ( Holoptychi u s, Fig. 2 5 3 ). 

 They are never, strictly 

 speaking, ' ganoid ' (p. 218); 

 but may be thick and shiny, 

 or thin and unpolished. In 

 the first case their exposed 

 surface is covered with a 

 layer of typical cosmine 

 (Fig. 1 90 and p. 217), which 

 in others seems to be 

 reduced, or to disappear 

 entirely, leaving the bony layers exposed (Fig. 249). The cranial 

 plates and the lepidotrichia agree in structure with the scales. 

 True denticles have not yet been definitely shown to exist. 



The proximal end of each lepidotrich extends inwards below 

 the body-scales, just as these overlap each other (p. 210). In 

 Odeolepis the scales overlap the dermal ray to a slight degree, in 

 Glyptolepis for a considerable length, still further in Ghjptopomus, 

 and finally, in Holoptychius the unjointed proximal segment of the 

 lepidotrich is almost as deeply embedded as in Dipnoi (p. 232). 



The orbit is small and situated far forward. The anterior 

 cranial bones (premaxillae, nasals, ethmoid, and even frontals and 

 prefrontals) tend to fuse to a continuous shield covering the snout, 

 especially in the Osteolepidae (Figs. 250, 257). It is the presence of 

 this shield, sometimes with paired notches below, which leads to 

 the conclusion that the nostrils were ventral, unlike those of living 



Fig. 240. 



Anterior edge of the dorsal fin of Eustlienopteron 

 Foordi, Wlit. ps, base of dermal ray ; /', endoskeletal 

 radial; sc, scale ; si, thickened scale-like segment of 

 anterior dermal ray. 



