186 PROCEEDINGS OF THE ACADEMY OP [1881. 



comparatively much larger. When the second and third radials 

 are short, the number of interradials decreases, and it increases 

 when they are long and narrow. This increase in number takes 

 place by interpolation, contrary to the radials in which the increase 

 is from the distal ends of the rays. The interradials are designed 

 to fill up vacant spaces in the test, and this accounts to some 

 extent for the great diversity which is found in their number 

 within the limits of a genus, and within species at different stages 

 of maturity. The first interradial is always larger than any of the 

 rest, and is situated between the upper sloping margins of the 

 adjoining first radials, except in some species of the Rhodocrinidse 

 in which it rests directly upon the basals, separating the ring 

 completely. There are generally two plates in the second series, 

 and two or three in each succeeding one, but in forms where the 

 secondary or tertiary radials form an arch over tlie interradial 

 spaces, as in Batocrinus, there is often only a single plate in the 

 second or third series. The plates decrease in size upwards, 

 those of the uppermost row being sometimes barely visible to the 

 eye. 



The posterior or anal area is readily recognized in most of the 

 Sph.Troidocrinidse by its greater width, and by having a larger 

 number, and a somewhat diflerent arrangement of plates. In 

 most of the genera the first anal plate is in line with the first 

 radials, resting upon the basals. In our remarks upon the 

 Cyathocrinidfe we noted the fact that in that family the anal plates, 

 with a few exceptions, are directed toward the right side. In the 

 Sphffiroidocrinidse, on the contrary, the symmetry is always bi- 

 lateral, in some cases almost perfectly pentahedral, and a vertical 

 section through the median line of the anal area, the anal aper- 

 ture, the central dome plate, and along the anterior ray, divides 

 the body invariably into two equal parts, and this symmetry, which 

 extends to the arrangement of the arms, is one of the most char- 

 acteristic features of the fiunily. 



The term "anal plates," as now used, designates the entire 

 series of plates which compose the posterior interradius. Properly 

 speaking, however, this is not quite correct. Careful examina- 

 tion shows clearly that the majority of these plates are in the 

 true sense of the word " interradials," while only a few of them 

 are actuall}' " anals," by which we understand plates supporting 

 the anus, or that can be accounted for as being in any way, directly 



