﻿70 SMITHSONIAN MISCELLANEOUS COLLECTIONS [vol. 47 



kind, but the evidence at hand seems sufficient to show that the 

 number of arms is of value when taken in connection with the other 

 features, yet it cannot be relied upon as the sole characteristic by 

 which species are to be distinguished. 



Wachsmuth and Springer 1 have noted that the number of arms 

 cannot be used for specific separation in the genus Dizygocrinus, and 

 this appears to be true also of Dolatocrinus. 



DOLATOCRINUS COSTATUS n. sp. 



(Plate XVI, 6, 6a.) 

 Description. — This is a large species with a low dorsal cup about 

 two and one-half times as broad as high. Base deeply indented, the 

 depression including all, or nearly all, of the radials. The sides of the 

 dorsal cup bend abruptly upward and stand at right angles to the 

 base. They are slightly constricted below the arm bases. 



The surface is ornamented by thick, somewhat irregular ridges 

 diverging from the center of each plate to the middle of its sides. 

 The center of the first interbrachial is marked by a circular pit, or 

 depression, from which the ridges pass to the sides of the plate. Dis- 

 tinct nodes appear to be absent from the surface, though the center of 

 a plate may be elevated by the intersection of the strong ribs. 

 Suture lines between the plates depressed. 



Basals not observed. Radials five ; costals two times five ; distichals 

 one time ten; palmars two times twenty. The first interbrachial is 

 the largest plate of the calyx, and is succeeded by two small plates 

 placed one above the other. A variable number of minute inter- 

 palmars is between or just below the arm bases. Arm openings 

 four to the ray and vertical in position. Interradial and interbrachial 

 pores four to six. Calyx slightly indented between the rays. 



Tegmen but slightly elevated, its plates large and smooth with 

 beveled edges. Anal tube subcentral. The surface of the tegmen 

 is slightly depressed between each pair of arms, the interradial de- 

 pressions being deeper than the others. 



Remarks. — This species is most nearly related to D. icosidactylus 

 Wachsmuth and Springer, from which it differs in the flatter tegmen 

 with its smooth surface and faint interradial depressions, the absence 

 of nodes on the surface of the dorsal cup, and the deeper basal ex- 

 cavation. 



Formation and locality. — Middle Traverse or Alpena limestone r 

 Collins' quarry, Alpena, Mich. 

 Cat. No. 26,396 l'. S. X. M. 



1 North American Crinoidea Camerata, p. 414. 



