TAX0CRIN1DAE 41 1 



iii regard to these can usually be settled by taking into account the decidedly restricted 

 number of interbrachials in Parichthyocrinus, and the close arching of the lower brachials 

 over them, and also a peculiarity of the right posterior radial, which in all the species, and in 

 some the left posterior also, is much smaller than the radials of the anterior and lateral rays ; 

 it is crowded by excessive growth of the posterior basal until it has much the form of the 

 primitive radianal in some of the Silurian genera. This is not the case in Forbesiocrinns, 

 where that radial is of full size and is usually larger than the succeeding primibrachs ; it will 

 also usually be found that the larger basals in Forbesiocrinus will determine the matter. 



A characteristic of this genus is the strong construction of its calyx walls and brachials, 

 producing with the closely folded arms a firm, rotund body, which retained its form under 

 pressure in fossilizing better than any others in this family ; in this respect also it is more 

 like the Ichthyocrinidae and Forbesiocrinus. This may be seen in the cross-sections of 

 various plates shown on Plate LXI; although all very short, they begin to have great 

 depth in the primary brachial series, and at the height of the tertibrachs they are at least 

 three times as deep as they are long. These characters very naturally led to the reference 

 of the first discovered species to Ichthyocrinus — the structure of the anal side, which would 

 have precluded such a course, not being disclosed by the specimens then at hand. 



Four species from two different horizons confirm the proposal made in 1902 for a 

 separate genus for this form. Its stratigraphic range is not large, the genus being con- 

 fined to the middle of the Lower Carboniferous, that is, so far as known, to the Upper 

 Burlington and Keokuk groups. 



The discrimination of species is necessarily made upon characters of a low order of 

 value, aided by their stratigraphic position. The original descriptions, as is the case in 

 many other genera, afford little assistance, being chiefly confined to statements of characters 

 now held to be of generic or family rank, and of individual peculiarities in the specimens. 

 The species may be conveniently arranged as follows : 



The Species of Parichthyocrinus 

 I. Anal tube bordered by weak perisome. 



Rays not prominent ; arms broad and flat. 



iBr few, in 2 or 3 ranges, mostly in vertical succession. 



Crown higher than wide P. nobilis. 



Rays prominent ; arms rounded. 



iBr few, mostly large plates in succession for 2 ranges, followed by 

 small ones at apex. 



Crown wider than high P. meeki. 



iBr doubling in second range, making wider areas of 3 to 4 large plates. 



Crown higher than wide P. subovatus. 



II. Anal tube bordered by strong plates, — transition toward Forbesiocrinus. 

 Rays not prominent ; arms broad and flat. 



iBr numerous in several ranges P. crawfordsvillensis 



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