§ 45. SHELLS OF THE CARBONIFEROUS SYSTEM. i OO 



Freshwater bivalve shells, comprising about ten nuvia- 

 tile species, occur in some of the coal-measures, forming 

 regular layers of limestone, called by the miners mussel- 

 bands, from the shells (Uniones), of which they are 

 chiefly composed. The marine tribes are in a great mea- 

 sure confined to the limestone below the coal. But in 

 Yorkshire, Professor Phillips has discovered a remarkable 

 exception ; in the coal-measures of that county, there is a 

 thin layer of marine shells, intercalated between fresh- 

 water strata. 



Many species of simple spiral univalves abound in some 

 of the limestones; and one genus, Euomphalus (Lign. 168, 

 fig. 5), is remarkable, from the inner volutions being tra- 

 versed by imperforate septa.* 



Numerous bivalves of existing genera of mollusca also 

 occur ; and the Pecten. or scallop, first appears in this 

 system in great numbers, comprising upwards of seventy 

 species. 



But the most striking modification in the molluscous 

 fauna, is the abundance of those extinct types of brachio- 

 poda (ante, p. 564), the Spirifers, Lept&nce* &c. of which 

 we first obtained a glimpse in the Permian system. These 

 occur in profusion, and entire beds of the mountain lime- 

 stone are conglomerates formed of the shells of these remark- 

 able animals. 



Sp'u'ifers. As we descend to the more ancient rocks, we 

 shall find these fossils yet more prevalent : and I will, there- 

 fore, in this place offer a few remarks on the structure of this 

 family of mollusca. The small subglobular bivalves (Tere- 

 bratuke) so abundant in the chalk, are sometimes found 

 empty, and if the valves be carefully separated, two curious 

 appendages are seen projecting from the hinge into the inte- 

 rior of the shell; these processes are the internal skeleton for 

 the support of the organs of respiration. t In the Spirifers 



" Medals of Creation, p. 426. f Ibid. p. 37S. 



