254 The American Naturalist. [March, 
When the changes of environment became too rapid, the forms 
either ceased, to exist or retrograded, became depauperate and 
finally extinct. Admirable illustrations are found in Batocrinns, 
Dorycrinus, and especially in the Hexacrinoid genus Dichocrinus. 
(3.) Variation may go on in one portion of an organism with- 
out materially affecting other parts. This is well shown in Ste- 
ganocrinus as compared with Actinocrinus, and among the 
the Platycrinide in Eucladocrinus and Platycrinus, in the Rho- 
docrinidz by Gilbertsocrinus and Rhodocrinus. 
(4.) The Actinocrinide show a decided tendency throughout 
their existence to increase the distal extent of the rays. In some 
forms it was accomplished by the simple branching of the free 
arms, as in Megistocrinus, certain Amphoracrini, and a few 
Actinocrini; by the lateral expansion of the arms, as in Eretmo- 
crinus ; or by radial extension of the calyx branchials, as, notably, 
in Teleiocrinus, Strotocrinus and Steganocrinus. The number of 
free arms was thus increased from twenty or thirty in the earlier 
species of Actinocrinus, to forty to sixty in Teleiocrinus, one hun- 
dred to one hundred and twenty-five in Strotocrinus, and from 
one hundred and fifty to two hundred in Steganocrinus. 
EXPLANATION OF FIGURES. 
PLATE VII.—Graphic representation of the generic relations and distri- 
bution in time of the American Actinocrinidze 
PLATE VIII.—Dorsal surfaces of Asunoctuide. B, basals ; 7, radials ; 
b, branchials; 4, free arms; z, interradials; am, primary anal plate; Z, 
pinnucles ; a, interaxillaries. 
1. Batocrinus pyriformis (Shumard). 2. Actinocrinus proboscidalis, 
(Hall.) 3. A. multiradiatus (Shumard.) 4. Teleiocrinus umbrosus (Hall.) 
5. Physetocrinus ornatus (Hall.) 7. Strotocrinus regalis (Hall.) 8. Stega- 
nocrinus sculptus (Hall.) 
PLATE IX.—1. Megistocrinus evansi (O. and Sh.) 2. Agaricocrinus 
wortheni (Hall.) 3. Free arm of Eretmocrinus remibrachiatus (Hall.) 


