82 FISH-KEMAINS OF THE CARBONIFEROUS ROCKS. 



dog-fish these plates are small and close-set, and form what 

 is called "shagreen"; in some of the rays, like the thorn- 

 back skate, they form large dermal tubercles. Lastly, the 

 skeleton, including the skull and vertebral column, is gene- 

 rall}'- gristly or cartilaginous, and therefore readily decom- 

 poses. Hence the parts which are susceptible of fossilization 

 and preservation in such a rock as our carboniferous lime- 

 stone are the teeth and scales, and the spines, which some- 

 times support the dorsal fins, as in the Port Jackson shark, 

 or project from the hinder part of the head. 



We may therefore picture to ourselves the fishes of the 

 mountain or carboniferous limestone period as shark-like or 

 ray-like forms with broad mouths, armed with sharp or 

 crushing (palatal) teeth, with shagreen-like skins, and some- 

 times armed with strong spines (ichthyodorulites). They 

 seem to have chiefly abounded in the earlier times of the 

 lower limestone shales (lower transition beds between the 

 old red sandstone and the carboniferous limestone) and in 

 the lower limestone. In the uj^per part of the limestone and 

 in the upper limestone shales (or upper transition beds be- 

 tween the carboniferous limestone and the millstone grit), 

 they are far less numerous, and are very rare in the millstone 

 grit itself. The fish palates have chiefly been found in a 

 special bed (the palate bed) in the lower limestone shales 

 just above the so-called Bryozoa bed ; and in special bands 

 in the black-rock limestone. We know that the lower car- 

 boniferous limestone sea was crowded with encrinites, and 

 may surmise that the ancient plagiostomes browsed on these 

 •old sea-lilies, the indigestible parts of which passed through 

 them and contributed to the calcareous matter of the lime- 

 .stone. Perhaps the fishes of the upper limestone period 

 browsed in a similar way on the abundant lithostrotion 

 corals. 



