108 THE PALEONTOLOGY OF MINNESOTA. 



[Distribution. 



GEOLOGICAL DISTRIBUTION. 



It is a singular fact that no remains whatever of Bryozoa are known from rocks 

 of earlier date than the Chazy limestone of the Lower Silurian System. Here the 

 class suddenly leaps into a prominence, not only in the way of individual represen- 

 tation, but in the matter of diversity of structure, that is both surprising and difficult 

 of explanation. Nor was it, as might be expected, the simpler types that prevailed 

 here. On the contrary, it is the more complex types like the Trepostomata and Cryp- 

 tostomata that are the most abundant and diverse in their development. What may 

 be even more surprising is that every suborder known in the fossil state was repre- 

 sented before the close of the Lower Silurian era. 



The vertical range of a few of the Lower Silurian genera (Stomatpont and Beren- 

 icea), is likewise remarkable, and not equalled, so far as known, in any other class of 

 animals, excepting the Brachiopoda, of which the genus Lingula, the same as the 

 bryozoan genera alluded to, has living representatives. Still, as a rule, the vertical 

 range of Bryozoa is restricted to comparatively narrow limits, and most genera and 

 many families fail to pass from one system of rocks to the next. 



LOWER SILURIAN SYSTEM : As has been stated, true Bryozoa are first met with 

 in the Chazy rocks of this system. In this group, excepting some of the calcai-eous 

 strata in New York and Canada, originally referred here, the conditions were often 

 quite unfavorable, not only for their preservation but for their development as well. 

 In the excepted beds several species of Phylloporina and Rhinidictya belonging to 

 the Cryptostomata, a considerable number of mostly undetermined Trepostomata, and 

 Mitoclema, a genus of the Cyclostomata, have been found. Following the rocks west- 

 ward from Canada the calcareous beds are lost, but the arenaceous portion, there 

 known as the St. Peter sandstone, a formation totally unfitted for their preserva- 

 tion, increases in thickness, and in Minnesota seems to be the only representative of 

 the formation. The marble beds at Knbxville, Tennessee, which probably belong to 

 the Chazy, are full of the remains of Trepostomata, none of which have, as far as we 

 know, yet received critical study. 



Following the Chazy are the Birdseye and Black River limestones and shales. 

 The first of these divisions has a wide geographical distribution, being known from 

 New York and Canada to Tennessee and Kentucky, as a fine-grained, massive or in 

 parts somewhat shaly limestone. The shaly layers are full of Bryozoa, among 

 which the Cryptostomata are preeminently developed. In Minnesota the greater part 

 of the " Trenton limestone " and the lower two-thirds of the shales resting on it, are 

 probably equivalent strata. Here the limestone is comparatively barren of Bryozoa, 

 but the shales, on the contrary, are exceedingly rich, affording also a greater diversity 



