(550 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



Only rarcl_y arc tlie synarthrial tubercles noticeable, while articular tubercles only 

 occur in robust individuals of Thaumatometra tenuis, the largest species in the subfamily. 



In all the included species syzygies occur between brachials 3+4, 9+10, and 

 14 + 15; the distal intersyzygial interval is from 2 to 6 muscular articulations, 2 in 

 about 44 percent, 3 in about 30 percent, and 4 in about 17 percent; only exceptionally 

 and by variation do 5 and 6 occur. 



Pi varies from about 10 to as much as 28 percent of the arm length, averaging 

 18 percent, or nearly a fifth; the number of segments varies from about 10 in some 

 small species of Thaumatometra, possibly also in the species of Bathymetra, to 42 in 

 Hathrometra sarsi, the average number being 20. The pinnule is usually very slender, 

 with all the segments but the fii'st 3 or 4 greatly elongated. Usually from 2 to 4 of 

 the basal segments are no longer than broad; but in some species only the first seg- 

 ment is no longer than broad, while in Boleometra clio and Thaumatometra tenuis from 

 5 to 7 of the basal segments are broader than long with their corners cut away so that 

 the pinnule, which in these forms has up to 35 segments, resembles somewhat the 

 coiTesponding pinnule in the Heliometrinae. The outer segments of P, are always 

 more than twice as long as broad, sometimes, as in Boleometra, not much more, but 

 more often, as in Hathrometra and in Trichometra, excessively elongated. The longer 

 segments almost always have more or less prominent spinous distal ends. 



Pi is longer than P2 in about 60 percent of the species in which the pinnules have 

 been described, varying from only slightly longer to a little more than 3 times as 

 long and being mostly from a third to a half again as long. In about 20 percent of 

 the species it is of the same length as P2, and in the same number of species it is 

 somewhat shorter. 



P2 when shorter than Pj, as it usually is, consists of from 8 to 20 segments, averag- 

 ing 13, and has up to 15 segments less than the corresponding Pi. When of the same 

 length as Pi, P2 usually consists of about the same number of segments. Po is much 

 more uniform in structure throughout the group than Pi, and usually resembles P3. 

 In only 3 or 4 cases is it intermediate in character between Pi and P3. In over a third 

 of the species it bears a gonad. 



P3 is nearly always a genital pinnule, but in Fariometra parnda and Thaumatometra 

 isis the first genital pinnule is P4 or Ps. The genital pinnules are quite uniform through- 

 out the group, varying only slightly in the number of segments, their relative length 

 and the development of spines on their distal ends. There is a sudden departure 

 from the usual structure, however, in Phrixometra, in which the genital pinnules of the 

 females bear a pouchlike marsupium. 



The distal pinnules are always very slender with all but the first 2 segments, which 

 are short and broad, much elongated with more or less spinous distal ends. They are 

 usually as long as Pi and consist of from 15 to 20 segments. 



The calcareous deposits in the ambulacral lappets most commonly consist of long, 

 slender or stout more or less curved or bent rods with the outer ends usually expanded 

 and spinulose and often pierced with a few holes. Thej^ may, however, be entirely 

 absent. 



The tentacles commonly contain a line or band of spicules along their outer side 

 which may be inconstant, reduced to a few scattered spicules, or altogether absent. 



For the details of the ambulacral deposits see vol. 1, part 2, pp. 274, 275. 



