458 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



Bathymetrical range. From 60 to 1040 meters. 



Thermal range. From 9.67 C. to 23.33 C. 



Remarks. The species of this subfamily are characteristic of water of from 100 to 

 300 meters in depth and with a rather high temperature, between 12 and 20 C. [NOTE 

 BY A.M.C.: Nanometra johnstoni from the Bass Strait, south of Australia, is probably 

 an exception to this. Unfortunately no bottom temperatures were taken in this 

 vicinity by the B.A.N.Z.A.R.E.] None of them are littoral, yet all of them occur 

 in less than 255 meters, and only a single one extends downward beyond 400 meters. 



Characters. The centrodorsal in the Perometrinae shows relatively little variation. 

 It is small, conical with the sides somewhat swollen, distinctly broader at the base than 

 high, more rarely nearly as high as broad; in small specimens it may be almost low 

 hemispherical. The bare dorsal pole is always small, roughened or papillose; it usually 

 forms a rounded point, but may be concave. The sides of the centrodorsal are almost 

 completely covered by several transverse alternating rows of closely crowded cirrus 

 sockets which decrease in size from the periphery toward the apex; about the periphery 

 there are approximately 15 sockets, or 3 beneath each radial. 



Like the centrodorsal, the cirri show comparatively little variation. They are 

 from 10 to 30 mm. in length, or from 25 to 55 percent of the length of the arms; in all 

 but 2 species they are very nearly one third of the arm length. 



The number of the cirri varies from XV to LXX, but is usually from XXV to 

 XXXV; in only one species are there more than L. 



The number of segments in the fully developed cirri is singularly restricted ; it varies 

 from 28 to 55, but is mostly between 30 and 40; it probably does not fall below 30 in 

 any species, and appears always to exceed 40 in only one.* 



The cirri always decrease in size from the periphery of the centrodorsal toward 

 the dorsal pole, and in extreme cases the apical cirri may be exceedingly delicate, 

 scarcely more than a tenth the length of the peripheral, and composed of about half 

 as many segments, all of which are much elongated ; but the difference is usually much 

 less than this. 



The cirri are practically uniform in shape. They are moderately stout, rather 

 strongly attached to the centrodorsal. and are composed of a few short basal, several 

 elongated proximal, and many short distal segments. 



The distal ends of the cirrus segments are usually unmodified, or in the smallest 

 species may be slightly produced. The longest proximal segments are twice as long 

 as broad, longer in only a single small species, and more or less, but never strongly, 

 constricted centrally; the segments in the outer half of the cirri are about as long as 

 broad, becoming broader than long in the large species. 



The dorsal surface of the short distal segments is always more or less sharply 

 carinate in the median line and is usually elevated so that the dorsal profile of the outer 

 part of the cirri is scalloped, or serrate with broad teeth; but in one case the elevation 

 is so slight that there appears to be little more than small terminal dorsal spines. 



Basal rays are not ordinarily developed; but in one species the ends of partially 

 differentiated basal rays may be seen in the angles of the calyx. 



The radials are usually rather prominent, extending some distance beyond the 

 rim of the centrodorsal; but in the larger species they may be almost concealed in the 



*But see note on page 464. 



