A MONOGRAPH OF THE EXISTING CRINOIDS 77 



Later he said that by the low median dorsal carination of the arms this species 

 approaches Chondrometra rugosa and Glyptometra tuberosa. From the species first 

 mentioned it is, however, easily distinguished by much smaller size, by a low centro- 

 dorsal carrying only XIII-XX cirri, and by more cirrus segments, though the cirri arc 

 only half as long as those of Chondrometra rugosa. It differs from Glyptometra tuberosa 

 in having more cirrus segments which are provided, besides, with small but distinct 

 dorsal spines, by smoother arm bases, by PI having much fewer segments, and by the 

 smaller size. 



The description of the cirri and of the relation between PI and P 2 show that these 

 specimens belong to a species in the family Thalassometridae, not in the family Charito- 

 metridae, and they are quite indistinguishable from specimens of Parametra orion of 

 corresponding size. 



Gislen described Perissometra carinata as follows: The centrodorsal is conical, 

 3 mm. broad at the base and 3 mm. high, with distinct interradial crests. The cirrus 

 sockets are arranged in 10 columns. 



The cirri are XXI, 20-21, about 15 mm. long. The fourth segment is slightly 

 longer than broad; the sixth segment is half again as long as the distal width and is a 

 transition segment; from the distal part of this segment the segments are polished and 

 are provided with a dorsal spine. This dorsal spine is peculiar in appearance, as from 

 the eighth segment it transforms into a strong longitudinal carination, a notch occurring 

 between the carination of two successive cirrus segments. The segments from the 

 eighth to the penultimate are one-third again as broad as long. The opposing spine is 

 small but distinct. The curved terminal claw is about equal to the preceding segment 

 in length. 



The radials are concealed. The IBr, are four times as broad as long, free laterally 

 though close to their neighbors, with sharp distolateral angles. There is a well defined 

 median crest and on each side of the median line a couple of indistinct pits. The IBr a 

 (axillaries) are three times as broad as long, pentagonal, with a raised and slightly 

 spiny proximal border; there is a sharp median carination in the proximal two-thirds. 

 The IIBr series are 2, and have a median carination similar to that on the IBr series. 



There are at least 12 arms (one postradial series is broken) 65 mm. long. The 

 proximal portion of the postradial series is not flattened laterally. The carination 

 continues dorsally out to the distal portion of the arms, which is slightly compressed 

 laterally. The brachials as far as the tenth are discoidal. The width of the proximal 

 portion of the arms is 1.7 mm. The distal brachials are slightly thickened at their 

 distal ends. 



In three cases out of ten the first syzygy (between brachials 3+4) is replaced by a 

 synarthry. The distal intersyzygial interval is 7 or 8 oblique muscular articulations. 



PI is 6 mm. long with 14-15 segments which are squarish and slightly prismatic. P s 

 is 4.4 mm. long with 11-12 segments. P 6 is 3.2 mm. long with 7 segments. The distal 

 pinnules are 6.2 mm. long with about 16 segments. ' 



The disk has been lost. 



In another specimen the centrodorsal is 1.8 mm. broad at the base and 1.2 mm. 

 high. The cirri are partly broken. The dorsal carination is weaker than in the speci- 

 men just described, more protruding in the distal part of the segment. The radials are 

 six times as broad as long. The IBr t are three times as broad as long, with a weak 



