206 HALF AN HOUR WITH SHELL-FISH (BIVALVES). 



Warwickshire, Staffordshire, Yorkshire, Lancashire, 

 and various other localities, you may see the lime- 

 stones, hundreds of feet in thickness, belonging to 

 the Silurian and Carboniferous formations, crowded, 

 nay, actually made up with the remains of fossil 

 shells. You examine these more minutely and find 

 that they belong to a class which, although still 

 living, seems to be gradually verging towards ex- 

 tinction. At least it occupies nothing like the 

 cosmopolitan distribution and abundance it enjoyed 

 in the earlier seas of the globe. This class goes by 

 the name of Brachiopoda, or "arm-footed," on 

 account of a certain internal structure to which we 

 will presently call attention. "We have two species 

 existing in British seas, but they are not common, 

 and only found at great depths. Until very lately 

 their occurrence was most rare, and a specimen was 

 worth considerable money value. Since deeper sea 

 dredging became popular, and marine naturalists 

 more numerous, these shells have turned up in 

 greater abundance, so that they are not so rare, 

 even in our smaller museums, as they formerly were. 

 These species are Terebratula caput serpentis, com- 

 monly called the " Snake-head " Terebratula, and 

 the Crania norvegica. The former is a delicate, 

 finely striated shell, of a greyish-white colour, and 

 about half an inch in length. Perhaps this is the 

 oldest species of mollusc in existence, for our best 

 Palaeontologist, Mr. Davidson, believes it to be 

 identical with a fossil species met with in the chalk 



