BRACHIOPODA 4!) 



complete series of " medals," or characteristic signs for the 

 identification of strata. The duration of types and species, as 

 a general rule, is inversely proportional to rank and intelli- 

 gence. The most highly organized fossils have the smallest 

 range, and mark with greatest exactitude the age of the deposit 

 from whence they have been derived. But the evidence 

 afforded by shells, if less precise, is more easily and constantly 

 obtained, and holds good over larger tracts of country. 



Class I. — BRACHIOPODA. 



The lamp-shells (Brachifrpoda), more than any other group, 

 have suffered with the lapse of time. Of 1300 known species, 

 only 75 are living ; and of the 34 genera, the larger part (21) 

 are extinct. The number of generic forms is greatest in the 

 Devonian period and least in the upper oolites, after which a 

 second set of new types gradually appears. The preponder- 

 ance of fossil Brachiopoda is contrasted with the scarcity of 

 the recent shells even more strongly by the abundance of indi- 

 viduals than by the number of species ; for the living shells 

 mostly inhabit deep water and rocky situations inaccessible to 

 the dredger, and are seldom obtained in large numbers. 



The genus Terebratula, as now restricted to shells with a 

 short internal loop, musters above 100 fossil species, of which 

 only one survives (T. vitrea), an inhabitant of the Lusitanian 

 province. The Waldheimias, or Terebratulce with long loops, 

 are widely distributed in our present seas, although only nine 

 living species are knov. n ; individuals of one or more of these 

 are found on the coast of Spitsbergen and Labrador, at Cape 

 Horn, and most abundantly in New South Wales and New 

 Zealand : there are sixty fossil species dating from the trias. 

 The Terebratettce, having the loop fixed to a mid ridge, com- 

 menced in the lias, and occur in small numbers throughout 

 the cretaceous and tertiary periods, and are the only lamp-shells 



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