1879.] NATURAL SCIENCES OF PHILADELPHIA. 313 



tricose above the arm bases. Calyx low, neatly ornamented by 

 radiating striae ; symmetry bilateral. 



Underbasals three instead of five, two of them equal and twice 

 as large as the third ; the smaller one placed anteriorly, the 

 sutures of the two larger directed to the rear. The plates are so 

 arranged that a bisection of the latter two produces five equal 

 pieces regularly alternating with the succeeding ring of basals. 

 Basals large, four of them pentagonal or hexagonal with sharp 

 upper angles, the fifth longer, hexagonal, truncate above. Radials 

 semi-lunate, larger than the basals, wider than long; articulating 

 facet concave and occupying about one-half the width of the plate. 

 The brachials consist either of a single bifurcating plate, which is 

 wider than long, pentangular; or in some species of two smaller 

 plates which combined have about the form and proportions of 

 the single one. 



Arms long, bifurcating, branches very numerous and of almost 

 uniform thickness throughout their length. The divisions take 

 place at regular intervals, but only toward the inner side of the 

 ray, the outer or main arms remaining straight; the inner arms 

 or branches remain simple or in some cases subdivide once or 

 twice. The first bifurcation of the main arms takes place upon 

 the first or second plate above the brachials, the next and all suc- 

 ceeding bifurcations from each first, second, or fourth plate, accord- 

 ing to species. Arm-joints distinctly rounded on the exterior, 

 with parallel sutures, rather higher than wide, the upper margin 

 generally projecting into a band inclosing the lower part of the 

 next plate. The axillary or bifurcating plates the largest, with a 

 sharp angular process at their sides toward the upper margin. No 

 pinnulse. Arm furrows covered by two rows of alternate plates 

 which stand out prominently forming an elevated arch. 



Anal plate one, which is large, resting upon the truncate basal 

 and supporting the ventral sac, which has the form of a tube. The 

 tube is long, sometimes extending beyond the tips of the arms, 

 rather strong; cylindrical except at its junction with the main 

 body, where it becomes much contracted toward one side ; the 

 inner passage elliptic, almost linear in its cross section. It is 

 composed of quadrangular plates longitudinally arranged, leaving 

 a deep furrow between the rows lined with fissures which penetrate 

 the test. 



The construction of the summit closely resembles that of Cya- 

 21 



