sciuciiERT.] GEOLOGIC DEVELOPMENT. 15 



unlit for marine life, and no bracliioj)ods are known from tbe Tertiary 

 de])()sits of this area. From the eastern North American Tertiary 9 

 species are known, but only 2 from the Pacilic binder. In recent times 

 conditions are apparently more favorable for the introduction and 

 existence of brachioi)ods from other areas, as li species have been 

 dredged from the Atlantic and 2i from the Pacilic continental plateaus 

 of North America. 



The living forms are universairy distributed in the seas ol' tlie world. Their range 

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

 and varying degrees of depth, from 200 to 600 fathoms being the usual limit of the 

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

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

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

 the CJudhiujcr expedition 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 110 species described, a 

 far greater development than in any other country. Davidson records 

 but 14 species in Great Britain, while Bigsby, in 1808, gave the total 

 for this system as 120 for all countries. In the next, or Ordovician, 

 system the rapidity of brachiopod differentiation is remarkable. There 

 are oil) 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 1808 he listed 550 Ordovician species, 

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

 Cambrian total of 120. 



While there is much specific differentiation throughout the Ordovi- 

 cian, it is a notable fact that the essential tyi)es of brachiopods 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 brachiopods 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 diflerentiation 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. ilag., Dec. IV, Vol. II, 1895, p. 3 (extract). 



