7 1 2 The Evolution of Forms from the Clinton [September 



charged with sedimentary matter by which they were entombed 

 as we now find them. 



Thus immense numbers of the cephalic and caudal shields of 

 Illeenus barriensis will be found in the space of perhaps ten or 

 fifteen inches, and outside of this accumulation there will not be 

 any except a stray one or so. 



In one vertical section of the stone in my collection, two inches 

 in diameter, the fracture shows thirteen shields of this trilobite 

 crowded one above the other. It also does not seem to have 

 any very regular lines of stratification. At Gasport the lime- 

 stone has a light bluish tint, and breaks, when comparatively 

 free from fossils, with a clean flinty fracture, and is very hard, 

 fine grained and compact. The majority of the shells have the 

 interior filled with crystallized calcite, and some of the larger 

 cephalopods are lined with crystals of the same.' From all infor- 

 mation obtainable, it seems that this layer has always been asso- 

 ciated with the Clinton group. Its fossils, however, prove that it 

 is more closely allied to the Niagara. 



The most common forms are Atrypa nodostriata and MirisUna 

 nitida, both of which are Niagara forms. After these we may 

 cite Spirifera radiata, /. ■ .•> and Illccnus barrien- 



sis ; which are common to both. The first of these reaches its 

 perfection in the Clinton, and is found in a minor degree in the - 

 subsequent shales of the Niagara, while the two last are but 

 sparingly found in the Clinton, and are found in the greatest 

 numbers at the opening of the Niagara series. 



But the most striking feature of this limestone — for which I 

 propose the name of the Niagara Transition Group — is the abun- 

 dance and perfection of the Cephalopoda, which in all other strata 

 of the Niagara period in Western New York are quite rare, with 

 the exception of Orthoccras annulatum, which is found in moder- 

 ate numbers in the Niagara shale and also is the most common 

 of the Clinton forms. In this respect as in the identity of a num- 

 ber of species, we find a strong analogy to the limestones repre- 

 senting the Niagara group in the Western States. In it we find 

 Cyitoccras he/rules, C. brevieome, Trochoceras eostatum, Troclw- 

 nema pauper, Palcsoeardia cordiformis, etc., which will be recog- 

 nized as western species. 



The majority of the species, as will be seen by the following 



