PART 5 A MONOGRAPH OF THE EXISTING CRENOIDS 601 



posteriorly directed tooth on the proximal border, on the other side with a distally 

 directed process of the preceding brachial. From the tenth onward the brachials pass 

 over from the markedly trapezoidal form to triangular which, however, is not always 

 fully attained. At the arm tips the brachials become longer than broad. 



Syzygies occur between brachials 3+4, 9 + 10, and 14 + 15, and distally at intervals 

 of 3 muscular articulations. The syzygial pairs are in general short, scarcely exceeding 

 the single brachials in length. 



P! is 7 mm. long, slender and filiform. The first seven segments are short with the 

 articulations constricted and the outer elongated, about twice as long as broad. P a 

 is similar, with 21 segments which increase in length toward the tip. The pinnule is 

 extremely slender, becoming as fine as a hair at the tip. The segments are in complete 

 and full contact with each other. In the middle of the pinnule the segments develop 

 dentate distal ends which overlap somewhat the bases of the segments following. 

 P 2 is 10 mm. long, with about 11 to 15 segments of which the two basal are short 

 and those following are greatly elongated; it is quite different in structure from P t . 

 P 3 is similar to Pj. On P 4 a decrease in the length of the segments is noticeable. It 

 is markedly shorter than the preceding pinnules. Sometimes there are 9 rather long 

 segments and sometimes there are 11 which are shorter than those in the 9-segmented 

 pinnules. The middle pinnules have about 12 segments which, with the exception 

 of the two basal, are elongated. The development of the gonads begins with P 2 on 

 which the gonad is on the fifth and sixth segments. Later it moves basally so that 

 on the pinnule of the twelfth brachial it is on the third to sixth segments and later 

 on the third to fifth. The segments of the genital pinnules are much more rounded 

 than are those of Hathrometra tenella and Poliometra prolixa. 



The perisome of the pinnules is without calcareous spicules. 



The disk is about 7 mm. in diameter. Closely placed sacculi are prominent 

 everywhere. 



The description of Psathyrometra acvta is as follows. The centrodorsal is sharply 

 conical, the sides in profile straight in the proximal two-thirds, thence very gradually 

 and slightly turning outward and running to the pointed tip, higher than broad, 2.7 

 mm. broad at the base and 3.5 mm. high, measured along the sides interradially. 

 The cirrus sockets are closely crowded all around the centrodorsal, which shows no 

 trace of division into radial areas. There are proxknally three columns of cirrus sockets 

 in each radial area; the two outer columns consist usually of 9 sockets which diminish 

 gradually in size from the base to the tip; the median column is incomplete, consisting 

 of 3 or 4 sockets only and ending slightly beyond the middle of the centrodorsal. 



The cirri are lacking. 



The ends of the basal rays are indicated by low, broad, and inconspicuous tubercles 

 in the interradial angles of the calyx. 



The radials are visible as curved bands with parallel sides from six to eight times 

 as broad as long just above the centrodorsal. The anterolateral angles of adjacent 

 radials are separated by a slight notch, the sides of which make an angle of about 90 

 with each other. There are no subradial clefts, but the line of junction between the 

 centrodorsal and the radials is slightly and narrowly incised. The IBrj are about 

 three times as broad as then* lateral length with the lateral edges, as viewed dorsally, 

 approximately straight and parallel and separated from those of their neighbors by a 

 narrow interval. The proximal border is straight, but the distal is deeply incised in 



