174 PALEONTOLOGY OF ILLINOIS. 



posed (Kev. of the Palaeocr., Pt. II, p. 96,) the group Bato- 

 crinites. If it was not for the fact that Dorycrinus immaturus 

 has but two arms in the anterior ray,* and Batocrinus mac-bridei 

 four, the two species could not be easily separated without see- 

 ing the posterior side of the calyx and the ventral covering. 

 The azygous interradius is much wider, and in the arm regions 

 forms a large gap between adjoining rays. At the four regular 

 sides, also, the interradials of the dorsal cup are continuous 

 with those in the vault, but the gap between those arms is much 

 smaller. Another small gap is found between the main divisions 

 of the rays; while the two arms comprising each division are 

 laterally connected with one another for quite a distance. The 

 pairs of arms are given off from a small bifurcating radial, each 

 arm having a separate ambulacral opening, but the openings 

 placed closely together. We allude to all these details to show 

 that Dorycrinus immaturus , and the two succeeding species from 

 the same horizon, have single arms, contrary to all later species 

 of Dorycrinus, in which, as a rule, two arms proceed from one 

 arm opening. The double-arm structure two arms from one 

 opening which we have observed only in the Batocrinites, was 

 introduced in the course of time from the single-arm structure, 

 as shown (Rev. of the Palaeocr., Pt. II, pp. 47-50,) in Bato- 

 crinus and Eretmocrinus, in which both forms are found fre- 

 quently side by side, and accompanied by most remarkable 

 transition forms. 



A comparison of the Le Grand species w r ith those in the Bur- 

 lington and Keokuk limestones, reveals the fact, that the former 

 have but half as many arms as the latter. Among the species 

 of the two beds the arms are distributed fundamentally on the 

 same principle, those, however, of the lower horizon have single 

 arms, while the others have two arms from each arm opening. 

 Another differentiation between the species of the different hori- 

 zons is shown by the fact that in species of the Burlington and 

 Keokuk the arm joints are extended into lateral spines; while 

 in the Le Grand species the tips of the arms are merely com- 

 pressed and flattened. 



* Since the above was written, we have found in a layer nine feet above the 

 main crinoidal bed a number of specimens of a variety of this species having uniformly 

 four arms to the anterior ray, but otherwise undistinguishable from it. 



