x. 1-3 OSTEOLEPIDS 269 



fins in the former. The paired fins have a characteristic scaly lobed 

 form, from which the group derives its name, and the skeleton of the 

 pectoral fin contained a basal element attached to the girdle and a 

 branching arrangement at the tip (Fig. 180). This plan is distinctively 

 different from that of the rayed fin of the Actinopterygii, but could 

 also easily have led to the evolution of a tetrapod limb (p. 307). 



The body was covered with thick, pitted rhomboidal scales, with an 

 appearance very similar to that of the palaeoniscid scale. These scales 

 have, however, a characteristic structure known as cosmoid. Each scale 

 (Fig. 145) may be considered as composed of an upper layer of dentine 

 (the cosmine), with a hard covering of shiny vitrodentine, possibly com- 

 parable with enamel. Below the cosmine is a 'vascular layer' consisting 

 of pulp cavities in which lay odontoblasts whose processes made the 

 dentine. This layer in turn rests on a bony layer of isopedin. The struc- 

 ture appears to have some relation to that of placoid scales and no 

 doubt the morphogenetic processes that give rise to the isolated pulp 

 cavities of placoid scales are similar to those that produce the cosmoid 

 plates. It is usual to suggest that the latter are formed of 'fused den- 

 ticles', but this is of course only a manner of speaking. Denticles do 

 not fuse, but morphogenetic processes may occur in such a way as to 

 produce flat plates of dentine. Indeed, it is possible that the evolution 

 occurred in the other direction, that is to say that the placoid scale 

 is a special case of the cosmoid. The condition in which a substance is 

 formed nearly uniformly all over the surface of the body is a more 

 general one than that in which such formation occurs only in isolated 

 areas. Indeed, the discontinuous arrangement is a very remarkable 

 condition, for which we have at present no explanation. The relation- 

 ship of the cosmoid to the ganoid scale of early Actinopterygii is not 

 quite clear, but the ganoid type seems to show a reduction of the 

 pulp cavities and development of the shiny surface-layer. The early 

 Dipnoi of the Devonian possessed cosmoid scales. In later osteolepids 

 and Dipnoi there has been a thinning of the scales, as among the 

 Actinopterygii, so that the later Dipnoi are covered with thin, over- 

 lapping, 'cycloid' scales. 



The skull of osteolepids (Fig. 161) was well ossified; there was a 

 series of bony plates arranged according to a pattern with a general 

 similarity to that of palaeoniscids (p. 230) and which might well have 

 been ancestral to that of amphibia. There was, however, a joint across 

 the top of the skull between the parietal and post-parietal bones, and 

 an unossified gap in the base of the skull. A movable joint at this level 

 persists in the living coelacanth (p. 272). 



