802 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



P. H. Carpenter employed two different methods in distinguishing the radial 

 and interradial parts of a crinoid. The ray opposite the anal area he frequently 

 designated as the anterior, while the other four he called right anterior, right 

 posterior, left posterior, and left anterior, respectively; but he also used a system 

 of lettering in which, viewing the animal from the ventral side, the anterior ray 

 was called ray A, the right anterior ray B, the right posterior ray C, the left 

 posterior ray D, and the left anterior ray E. According to this terminology the 

 anal area would be known as interradius C-D, while the two derivatives from each 

 radial in 10-armed types might be differentiated, as he suggests, as Aj and A,, 

 Bj and B 2 , etc. This system of marking the topography of the disk has a number of 

 advantages over that preceding, and will be employed in the succeeding pages. 



As was first noticed by P. H. Carpenter, the ventral perisome varies consider- 

 ably in thickness in the different groups. Generally speaking, that of the macro- 

 phreate forms is thin and delicate, and the ambulacra! grooves are but slightly 

 raised above it. In the oligophreate types the perisome of the disk is much thicker 

 and more opaque, and the ambulacral grooves are more or less elevated above the 

 general surface. 



In the endocyclic comatulids the mouth is usually situated very near (just 

 anterior to) the center of the disk (figs. 710-728, p. 346), especially in species in 

 which the disk is deeply incised (figs. 713-727, p. 346) ; but frequently it occupies 

 a more anterior position, having moved outward along radius A. This is most 

 noticeable in large species, or in large specimens of smaller species, which have no 

 incisions in the interbrachial margin of the disk, and is particularly noticeable in 

 the large species of Heliometrinse, and in Tropiometra (fig. 731, p. 346). It is 

 rare, however, for the mouth to be further removed from the center of the disk 

 than half the distance between the center and the base of ray A, and uncommon for 

 it to be even so much displaced. In the two most primitive endocyclic families, the 

 Pentametrocrinidsa (figs. 758, 759, p. 353, and 1158, pi. 25) and the Atelecrinidse 

 (figs. 1163, 1164, pi. 26), the mouth is almost exactly in the center, but it appears 

 never to occupy quite this position in any of the other groups. 



From the mouth there radiate outward across the disk five equal ambulacral 

 grooves, which branch dichotomously just before the arm bases are reached (part 1, 

 figs. 15, 16, 18, 19, p. 67), and farther out branch again with each division of the 

 post-radial series (part 1, fig. 17, p. 67). Ordinarily the branching of the ambu- 

 lacral grooves upon the disk takes place near the margin, and all the primary 

 ambulacral groove trunks are of equal length ; this is especially true in the Macro- 

 phreata (figs. 747-757, p. 349), but in certain genera belonging to the Oligophreata, 

 more especially in Tropiometra (figs. 729-733, p. 346) and in Ptilometra (figs. 739- 

 741, p. 349), the division of the primary groove trunks is irregular both in different 

 individuals and on the different rays of the same individual, sometimes occurring 

 at the peristome, so that, instead of the single primary trunks, two secondary trunks 

 are given off side by side. 



