scHucHERT.] GEOLOGIC DEVELOPMENT. 15 



unfit for marine life, and no bracliiopods are known from the Tertiary 

 deposits of this area. From the eastern i^ortli American Tertiary 9 

 species are known, but only 2 from tlie Pacific border. In recent times 

 conditions are apparently more favorable for the introduction and 

 existence of bracbiopods from other areas, as 14 species have been 

 dredged from the Atlantic and 24 from the Pacific continental plateaus 

 of North America. 



The living forms are universally distributed in the seas of the world. Their range 

 in depth is no less extended. They occur in shallow waters, at low-water mark, 

 and varying degrees of depth, from 200 to 600 fathoms being tbe usual liaiit of tlie 

 majority of species. Several far-ranging abyssal species were dredged in from 1,000 

 to 2,000 fathoms. The delicate transparent shell of that interesting little Terebratu- 

 loid, Liothyrina Wyvillei Davidson, was actually obtained in a living condition by 

 the Challenger exjiedition from the enormous depth of 2,900 fathoms, or 3^ miles, at 

 the bottom of the South Atlantic Ocean.' 



In the North American Cambrian there are IIG species described, a 

 far greater development than in any other countrj'. Davidson records 

 but 14 species in Great Britain, while Bigsby, in 18G8, gave the total 

 for this system as 126 for all countries. In the next, or Ordovician, 

 system the rapidity of brachiopod differentiation is remarkable. There 

 are 319 species known in North America, an increase nearly three 

 times that of the Cambrian. Bigsby's percentage of increase for this 

 system is even greater, since in 1868 he listed 556 Ordovician species, 

 which represent a growth of nearly four and one-half times that of his 

 Cambrian total of 126. 



While there is much specific differentiation throughout the Ordovi- 

 cian, it is a notable fact that the essential types of bracliiopods of this 

 system are also found near its base in the Calciferous. In the Chazy, 

 or next younger horizon, the species are very much like those of the 

 Trenton, where this class has great and varied representation, which 

 is maintained to the end of the Ordovician. It is also true that the 

 species become more generalized structurally as the Cambrian is 

 approached, and most rapidly so toward the base of the Ordovician. 



The evolution of the Cambrian bracbiopods is similar in its history 

 to that of the Ordovician, except that there the differentiation was 

 along more fundamental structural lines. In the following table it is 

 seen that the four orders of the class Brachiopoda began with the 

 Lower Cambrian, and that throughout this system differentiation was 

 mainly of family importance, since none of these divisions has many 

 genera or species. Where minor groups occur in quantity it is always 

 in the more primitive divisions, as in the Atremata. In none of the 

 other three orders is there a similar rapid differentiation in the 

 Cambrian. 



'Agnes Crane, Geol. Mag., Dec. IV, Vol. II, 1895, p. 3 (extract). 



