172 SMITHSONIAN INSTITUTION 



four stages by addition as shown in the following table, the upper line representing the pos- 

 terior rays and the lower the anterior : 



2-2 3-3 3-3 4-4 



2-2 2-2 3-3 3—3 



2222 



In Dizygocrinus similar modifications occur, the lesser number usually remaining in the 

 anterior ray. In most of the Devonian species of Megistocrinus, however, the anterior ray 

 participates in the increase, producing an irregularity similar to that of the present genus, 

 the 16 arm openings being distributed thus : 



4-4 

 2 — 2 



4 

 This subordination of the lateral rays is really a primitive condition relating to the cystids, and 

 is well shown in the remarkable cysto-crinoid genus Hybocystis, in which the lateral rays do 

 not develop any arms, but instead are provided with recurrent ambulacra. In certain Inadu- 

 nate genera, Pachylocrinus, Scytalocrinus, etc., the anterior ray frequently remains simple 

 while the others bifurcate ; and in some, e. g. Atelestocrinus, it is suppressed altogether. 



In all the foregoing cases the equality or predominance of the posterior rays has been 

 maintained, and it would seem as if this were in accordance with a fairly general rule. But 

 by way of further extension of the catalogue of variations, there remains for mention the most 

 curious example of all, where in an isolated species at the close of the Lower Carboniferous 

 the case of Cholocrinus is exactly reversed, — the two posterior and the anterior rays being 

 degenerate while the lateral rays are enlarged to double their size. This was described by 

 Whitfield x in 1891 as Cyathocrinus maxvillensis from three specimens found in the Kaskaskia 

 group. I have a much more perfect specimen from Grayson County, Kentucky, in the collec- 

 tion of the late Col. S. S. Lyon, who had prepared a description and figure of it under one of 

 his favorite hyphenated names, Poteriocrinus brachialis-irregnlaris (it is a Poteriocrinoid, 

 not Cyathocrinus). In this specimen the two enlarged lateral arms are preserved to their 

 full length ; they are long, ponderous, and simple, while the two posterior rays bifurcate on 

 the first primibrach into a pair of short and delicate arms about half as long as the others. 

 The occurrence of this peculiarity in four specimens from two distant localities removes it 

 from the category of abnormality, and proves it to be a definite structure. 



Aside from the inequality of the rays, Cholocrinus is interesting for the extreme irregu- 

 larity of the heterotomy, the ramules being of various sizes, about four to the ray, and not all 

 on the same side of the dichotom ; they are at various intervals, sometimes succeeding each 

 other on the same side, and sometimes alternating ; and they in turn divide again irregularly. 



Interest also attaches to the discovery of the tegmen in one of our specimens of Cholo- 

 crinus obesus, viz., the original of Angelin's plate 26, figure 6, already mentioned as affording 

 a distal view of the arms. This specimen consists only of the rays, from which the dorsal 

 part of the calyx is broken away leaving the inner floor of the tegmen exposed (PI. IX, 

 fig. 4-d) ; it is of the usual general type for this group, but has very strong ambulacra, some- 

 what like those of Onychocrinus, and is considerably different in detail from the tegmen of 

 Synerocrinus and Taxocrinus. 



Only the one species is known. 



1 Ann. New York Acad. Sci., vol. 5, p. 577, pi. 13, figs. 5-8. 



