﻿62 SMITHSONIAN .MISCELLANEOUS COLLECTIONS [\'OL. 47 



more elevated than in the latter species and is not depressed in the 

 interradial areas. From M. tuberatus n. sp. the present species differs 

 in the pointed spines of the surface, the higher tegmen, and the 

 absence of fine surface sculpture. 



The type specimen is imperfect, only half the crinoid being present, 

 as indicated by the diagram of its plates (fig. 2). The flattening of 

 the base represented by fig. 1, pi. xv, is due to crushing. The base 

 is normally but slightly flattened. 



The large specimen represented by fig. la, pi. xv, closely resembles 

 the type of M. sphczralis, and differs from it only in the arrangement 

 of its posterior plates. In this respect the specimen is unique. The 

 fused basals form a five-sided plate. The plates adjoining the basals 

 on three sides are normal radials, and the structure of the crinoid in 

 the anterior and antero-lateral rays does not differ from that of the 

 type specimen so far as can be determined. Between the basals and 

 radials of the postero-lateral rays are two small plates which, not- 

 withstanding their position, appear to belong rather to the anal series 

 than to the ray, since the otherwise normal ray is complete without 

 them. These plates are without ornament, but resting upon them 

 is a large plate ornamented like the radials. This plate is followed 

 by two and four plates in the succeeding rows. The higher plates 

 of the anal interray are few and small (see fig. 3). The whole num- 

 ber of plates in this area, thirteen, does not differ widely from that 

 of the type which has about sixteen plates. 



The arrangement of the calyx plates is considered of fundamental 

 importance in the classification of crinoids, and upon such evidence 

 alone this specimen would represent a new genus, but its close resem- 

 blance in other respects to three specimens undoubtedly of the genus 

 Megistocrinus leads the writer to consider it an abnormal variation, 

 and to place it, at least provisionally, with M. sph&ralis. The de- 

 velopment of such an abnormal individual may be accounted for if 

 we imagine a bisection of the young anal plate, the halves of which, 

 owing to some downward-acting force, grew laterally instead of 

 vertically, and became intercalated between the adjacent basals and 

 radials, while the middle plate of the second row moved downward 

 until it came to lie wholly below its neighbors. The cause of such 

 a downward acting force remains to be determined, but the tendency 

 for higher plates to occupy lower positions in the calyx is shown, 

 not only by such forms as Tylocrinus novus n. sp., described below, 

 but by the inclusion of higher brachials within the calyx as in the 

 ( lamerata. 



