ACTINOCEINIDiE. 637 



there is nothing between the two forms that could not be readily explained 

 by individual growth, except the great contrast in the size of the specimens, 

 which on the one hand are quite large, and on the other very small, the 

 intermediate forms being wanting. This in pai-t has led us to propose a new 

 name for the larger form ; but the principal reason is that it is practically 

 impossible to recognize the species, or even the genus, from Meek and 

 Worthen's description. 



STROTOCRINUS M. and W. 



* 



1866. Meek and Wokthen (includiug Teleiocrirms) ; Geol. Rep. Illinois, Vol. II., p. 188, and Proc. Acad. 



Nat. Sci. Pliila., p. 253. 

 187S. ZiTTEL (including Teleiocriiius and Phj/setocrlnus) ; Haudb. d. Palieont., Vol. I., p. 370. 

 1881. W. and Sp. ; Revision Palffiocr., Part II., p. 158 (Proceed. Acad. Nat. Sci. PLila., p. 332). 

 1889. S. A. Miller; Nortli Araer. Geol. and Palajont. p. 283. 



Sjn. Calathocrinits Hall (Subgenus of Actiiioeriims) , 1861; (not von Meyer 1818, Bronn's Jalir- 

 buch, p. 4G9). 



Strotooimts holds the same relation toward Pliysetocriniis that Teleio- 

 crinics does toward Cactocrinus ; the first two being distinguished by having 

 an anal opening within the disk, while the last two have an anal tube. In 

 the construction of the calyx as far as the top of the distichals, all these 

 forms differ only immaterially ; but from the palmars up in Strotocrinus, as 

 in Teleiocrinus, owing to the great increase in the number of arms, the 

 brachials were crowded outwards and formed a broad, continuous, ten-rayed 

 rim, in which not only the lower parts of the arms, but also the lower pin- 

 nules, became incorporated. The specimens are of very large size, and the 

 number of arms enormous, the disk sometimes attaining a width of 12 cm., 

 with as many as fifteen bifurcations in each division, or one hundred and fifty 

 arms altogether. The arms are given °oS alternately from opposite sides, 

 each order consisting of a single axillary plate, which supports at one side an 

 arm, a brachial of a higher order at the other, and the uppermost one two 

 arms. The arms are thin and comparatively short. Interbrachials not 

 numerous, and never extending beyond the palmars. Disk flat or very 

 slightly convex ; composed of thousands of minute, irregular plates, which 

 decrease in size as they approach the arms. Anal opening excentric, rarely 

 rising above the general plane of the disk. 



Distribution. — Restricted, so far as known, to the Upper Burlington lime- 

 stone of the Mississippi Valley. 



Tyjie of the genus : Strotocrinus rcgalis. 



