648 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



The cirri range in number from 10 to about 100, but are usually between 30 and 

 60. There is always some difference between those about the periphery of the centro- 

 doi-sal and those about the dorsal pole; when the cirri are very numerous the apical ones 

 may be only a third as long as the peripheral with not more than half as many segments, 

 which are more uniform and more generalized in structure. 



The cirri are always of moderate size, sometimes rather slender but never very 

 stout, and varj"^ from about 10 to 40 percent of the arm length. They are most com- 

 monly between 25 and 35 percent of the arm length, or from a quarter to a third the 

 length of the arms. 



As a rule they are rather dehcate and fragile and easily detached from the centro- 

 dorsal, especially in those species in which the centrodorsal is conical. They are always 

 laterally compressed, more strongly in the distal than in the pro.ximal portion, and are 

 sometimes, especially in some species of Thaumatometra, much flattened. 



The number of segments in the cirri varies from 8 to 10 in Bathymetra abyssicola, 

 10 in Thaumatometra septentrionalis, and 10 to 12 in Thaumatometra parva and in 

 Retiometra alascana, up to 40 to 45 in Trichometra vexator. The majority of the species 

 have between 15 and 30. 



The distal segments are always shorter than the earlier segments. The longest seg- 

 ments vary from about as long as broad to about 6 times as long as broad, and are most 

 commonly 2 or 3 times as long as broad. The distal segments are usually about as 

 long as broad, but may be broader than long, or when the cirrus segments are all much 

 elongated the penultimate may be as much as twice as long as broad. 



Generally the longer segments are more or less strongly constricted centrally 

 with expanded distal ends which overlap the base of the segment succeeding, this 

 feature dying away as the segments become shorter and giving place to a sharp 

 carination in the middorsal line which at the distal end may be produced into a spine. 

 Sometimes the distal edges of the long earlier segments are spinous. Rarely the 

 segments are neither enlarged nor overlapping at the ends. 



The radials in the Bathymetrinae for the most part are very short, extending 

 only verj^ slightly beyond the rim of the centrodorsal in the median hue and forming 

 low triangles in the interradial angles. In some large species, as in Boleometra clio, 

 they may be entirely concealed in the median line and only visible as low interradial 

 triangles, while in the small species a relatively greater proportion of the dorsal surface 

 is exposed. This reaches its maximum in the species of Bathymetra, in which the 

 radials appear externally as relatively large plates twice as broad as long in the median 

 line, extending far forward interradially where they separate the bases of the IBri. 



The radials are never carinate or tubercular in the median hue, and the distal 

 border is almost invariabl3f smooth, only very exceptionally showing a few fine spinulcs. 



The disk is almost uniform throughout. It is large, with the interradial borders 

 convex, straight, or slightly concave, and the surface is without visible calcareous 

 deposits. 



Sacculi are abundant along the ambulacral grooves of the pinnules, less numerous 

 along those of the arms, and still less numerous along those of the disk. 



The IBr series show a considerable amount of variation. The IBri are short, 

 usually from 3 to 5 times as broad as long in the median line, sometimes from 8 to 

 10 times as broad as long and rarely only twice as broad as long, usually with parallel 

 or more or less strongly convergent lateral borders and the distal border more or less 



