252 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



distance somewhat greater than its own length ; this process may originate from the 

 border of the side plate, but usually arises from just within the border, which pro- 

 jects over it like a shelf. In a transverse section of a pinnule the side plates are 

 seen to be strongly and evenly curved, the convexity being outward. In a ventral 

 view of the pinnule the outer edges of the side plates, toward the ambulacral groove, 

 are seen to be cut away by a semicircular incision, and the distal and proximal 

 angles are broadly rounded off so that the semicircular incision is bordered by two 

 converging plates resembling the horns of a crescent, of which the proximal extends 

 farther inward toward the middle of the pinnule than the distal ; in the border of 

 the semicircular incision proximal to the middle is an obtusely angular process, 

 which disturbs the symmetry of its contour. The sacculi, which are easily seen in 

 cleared specimens through the side plates, reach the surface through the semi- 

 circular incision of the outer borders — ventrally, therefore, instead of laterally, as 

 in Ptilometra. On the inside of the side plates are two prominent ridges, which are 

 parallel and perpendicular to the proximal border; one of these runs midway be- 

 tween the center and the proximal, the other midway between the center and the 

 distal edge. The former is the higher, bearing the covering plate on its outer end. 

 its inner end terminating against the process which extends over the inner side of 

 the pinnular. 



The covering plates are very broadly oval or somewhat kidney-shaped, broadei 

 than long. 



Asterometra macropeda (figs. 812-815, p. 385). — The side and covering plates 

 in this species do not differ essentially from those of A. anthus; but the process from 

 the base of the side plates which runs over the inner surface of the pinnule is 

 broader and trapezoidal with its free inner edge, which is parallel with the base of 

 the side plate, longest. 



Asterometra cn^stata. — The conditions in this species do not differ essentially 

 from those in A. anthus. 



Pterometra magnipeda. — The side and covering plates in this species resemble 

 those in Asterometra anthus. 



Pterometra spJendida- — The side and covering plates of this species resemble 

 those of Asterom^etra anthus, but the structure is much coarser. 



Pterometra pulcherrima. — The j^erisomic plates are essentially like those of 

 Asterometra anthus. 



Pterometra trichopoda. — The side and covering plates resemble those of Astero- 

 metra anthus. 



The perisome over the genital glands is completely covered by large filmy 

 plates. 



Cosmiometra crassicirra (figs. 843-848, p. 405). — In the plane parallel with the 

 dorsoventral plane of the pinnule the side plates are very strongly convex, so that 

 -when the pinnule is viewed ventrally they appear as a row of half cylinders with 

 the convexity outward. In lateral view the main body of the plat* is about as high 

 as broad ; the base upon which this portion rests is very narrow, and in contact with 

 the bases of the adjacent plates; the sides are throughout their whole length regu- 



