1885.] NATURAL SCIENCES OP PHILADELPHIA. 229 



There is, however, one exception to this rule, presented by the 

 remarkable genus Acrocrinus (PL 6, fig. 1), in which the basals 

 and radials ai'e separated by from four to fifteen rings of small 

 pieces, their number varying in species, and increasing in the 

 growing Crinoid. 



The plates of the basal ring are laterally connected except in 

 the two genera Zeacrinus and Calpiocrinus. In the former they 

 are small, trigonal, acuminate pieces, which externally, and also 

 at the inner floor of the calyx, are separated by the radials, which 

 with their truncated lower angle meet the underbasals. In Cal- 

 piocrinus four of the basals seem to be totally absent externally, 

 and only the posterior one is represented by a small quadrangular 

 piece. The underbasals differ considerably in size, and are fre- 

 quently covered entirely b} r the column. In such cases it is often 

 exceedingly difficult to distinguish them from the upper stem 

 joint. Several species have been described with underbasals 

 which do not possess them, and Heterocrinus and Glyptocrinus 

 were thought to contain species with underbasals and without 

 them. 



Considering the importance that has been given to the presence 

 of underbasals in classification, and the difficulty of identifying 

 them in some groups, it is of some importance, that we have 

 discovered a method, by which, in most cases, the presence or 

 absence of underbasals can be ascertained accurately from the 

 column, the position this occupies toward the general symmetry 

 of the cal}x ; from the outer angles of the stem joints, their posi- 

 tion and that of the cirrhi, whether these are radial or interradial, 

 and from the direction of the rays in the axial canal. The follow- 

 ing rules prevail : — 



1. In species with underbasals, whenever the column is pentan- 

 gular, its longitudinal angles are directed interradially, the sides 

 and columnar cirrhi radially ; on the contrary, in species with 

 basals only, those angles are radial, the sides of the column and 

 the cirrhi interradial. 



2. When there are underbasals and the column is pentapartite, 

 the five sections of the column are radial, the longitudinal sutures 

 interradial, the radiation along the axial canal radial ; but the 

 opposite is the case when basals only exist. 



For further particulars, we refer to our diagrams on Plate 6, 

 which represent species of widely different groups ; and we will 



