MONOGRAPH OF THE EXISTING CRINOIDS. 



91 



of which gives off internally, on every alternate ossicle, an undivided arm, eventu- 

 ally terminating in a pair of arms which, contrary to the case in Gomaster, are 

 not quite alike. In their method of division the arms of the species of the genus 

 N eocomatella agree with those of the smaller species of the genus Comatella. 



In the genera Capillaster (figs. 180, p. 92, and 181, p. 94) and Nemaster (fig. 

 182, p. 96) the division series beyond the second are always 3(2+3), this type 

 being derived from the more usual 4(3+4) by the omission of the first ossicle. 



The arms usually taper evenly and gradually 

 from the base to the tip, but often they slowly in- 

 crease in size from the base to the twelfth or four- 

 .teenth brachial and from that point slowly taper to 

 the extremity; this condition is especially prevalent 

 in the Oligophreata, though more or less character- 

 istic of many of the more specialized of the macro- 

 phreate forms, such as Antedon petasus or Helio- 

 metra glacialis. It is commonly correlated with the 

 degree of recumbency of the radials, reaching its ex- 

 treme form, in the recent comatulids, in those co- 

 masterids in which the radials lie horizontally. It is 

 particularly well seen in Comatulella l>rachiol<!ta 

 (part 1, fig. 77, p. 130; pi. 50, fig. 1332), Comatula 

 rotalaria, and 0. Solaris, though almost equally well 

 marked in many other types (fig. 184, p. 100) . 



In a number of recent comatulids, most marked, 

 perhaps, among the species of Comactiniinse, the 

 arms are so swollen in their lower portion that the 

 brachials do not reach entirely across them but 

 terminate in a point somewhere between the median 

 line and the outer margin. This is not due to a per- 

 sistence of the original biserial arrangement, as 

 might appear at first sight, but instead, as is shown 

 by the development of the individual, is a purely 

 secondary condition, a sort of pseudo-biserialism. 

 In the very young brachials the proximal and distal 

 edges are strongly oblique, and one of the lateral FlG - ITS- CROWN AND PBOXIMAL POB- 



j , , ,, ,, TION OF COLUMN OP SPECIMEN OF 



edges is much longer than the other, so that in dorsal MONACHOCBINUS SEXBADIATUS. 

 view the brachials appear to be trapezoidal in shape ; 



during growth material is added on each lateral border. As the ends always 

 maintain their original obliquity, addition of calcareous matter on the lateral 

 borders causes the longer side to become still longer and the shorter side to 

 become still shorter until it finally becomes a point beyond which the horizontal 

 (transverse) suture line between the proximal border of the succeeding and the 

 distal border of the preceding brachials runs to the edge of the arm. 



Usually, and always in the endocyclic species, all of the arms in a single 

 specimen are just alike, or at least the various arms arising from each radial 



179 



