154 CRINOIDS. 



the earlier species the free arms are slender, growing much 

 stouter in the Burlington and Keokuk, and in the latter often 

 also branching one or more times. This development is accom- 

 panied by an increasing massiveness of the calyx plates, and a 

 change of the simple convexity of the ossicles into great, rude 

 nodosities. Another marked feature is the tendency of the 

 rays to separate from one another above the second brachials, 

 forming prominent radial extensions before giving off the free 

 arms. At the^same time the interradial areas become consider- 

 ably depressed. The quinquelobate calyx is thus produced — 

 a form upon which the genus was founded. In general it may 

 be said that the earlier forms were of small size, delicately 

 constructed and ornamented, and that they gradually became 

 very much larger, more massive, with rough, rugged sculptur- 

 ing. 



The more striking points in the development of the anato- 

 mical features in Actinocrinus, as here briefly traced, apply to 

 the other genera just mentioned, and also to the members of 

 other related families. For example : Dorycrinus developed 

 huge ventral spines ; Batocrinus, an immense disk-shaped calyx ; 

 Eretmocrinus, broad, lanceolate arms ; Strotocrinus, a large 

 rim stretching out laterally from above the tertiary brachials; 

 and Steganocrinus, monstrous radial extensions, from which 

 the free arms sprung. 



The distinctive structural characters of the genera of Ac- 

 tinocrinidae and their general lines of development have already 

 been indicated. It now remains to allude briefly to the generic 

 relationships of the several groups. As previously stated, Pe- 

 riechocrinus and Megistocrinus are closely related, but they 

 differ considerably from other members of the family. Their 

 recorded history also extends over a much longer period than 

 that of the other twelve genera. Periechocrinus occurs first 

 in the Niagara — large, thin-plated forms, nearly devoid of orna- 

 mentation, and having tall, obcohical calyces, with long arms 

 branching one or more times. The evidence of this type in the 

 American Devonian is as yet rather meager, though in Europe 

 abundant testimony of its existence in rocks of that age is 



