MONOGRAPH OF THE EXISTING CRINOIDS. 259 



either end a stout process of the same thickness as the edge of the plate itself, which 

 extends inward, at right angles to the edge of the plate, for a distance equal to 

 about two-thirds the length of the latter. The figure presented by the inner end 

 of the plate as seen in end view is thus three sides of an oblong; the area included 

 between the edge of the plate and the inward extensions may be three sides of a 

 square, but is usually more or less rounded, sometimes about three-quarters of a 

 circle. In direct ventral view the side plates appear semicircular, with the proximal 

 inner end of the semicircle produced in a broad process which touches the distal 

 inner end of the semicircle formed by the preceding plate. 



The covering plates, which are almost circular, or slightly longer than broad, 

 and flattened at the base, are attached to the proximal outer process halfway be- 

 tween its tip and the slight angle marking its junction with the main body of 

 the plate. 



There are no deposits in the tentacles. 



Perissometra selene (figs. 817, 818, p. 385, and 819, p. 392). The side and cover 

 ing plates resemble essentially those of Crossometra helius. The distal process 

 on the side plates is much thicker at the base, and the angle on the outer border 

 at the base of the broad proximal process is much more developed. Within the 

 semicircular notch, as the plates are viewed from an angle of 45 to the dorso- 

 ventral plane of the pinnule, between the posterior process and the median angle 

 there is a very small irregular plate, often of only two or three meshes. Toward 

 the base of the pinnule, alternating with the bases of the side plates and lying 

 deeper in the perisome, are irregular plates of considerable size with ragged edges 

 representing a second, deep, row of side plates. On the proximal side of the first 

 two pinnulars normal side plates, much reduced in size, alternate with side plates 

 without covering plates in the form of equilateral triangles which reach to the 

 level of the bases of the covering plates on the normal side plates. The normal 

 side plates and the covering plates which they bear rapidly increase in size and 

 after the third or fourth the secondary side plates pass deeper into the perisome 

 and become rudimentary, persisting to the fifth pinnular. 



The covering plates are nearly circular and less flattened than those of Crosso- 

 metra helius. 



Perissometra patula. The side and covering plates do not differ essentially 

 from those of CrossoTnetra helius, but they are coarser in texture, and the side 

 plates have ragged borders. 



Perissometra lata. In lateral view the side plates, which are of the same 

 structure as in related species, appear more nearly rhombic, as they are very close 

 together and the large proximal outer processes of each are covered by the distal 

 processes of the following; the notch in the distal border is very small and often 

 very shallow. The concavity between the distal outer process and the median angle 

 of the outer edge contains a very small more or less irregular plate. 



On their lower side the covering plates bear a low keel in their proximal third 

 which runs somewhat diagonally. 



Pachylometra distincta (figs. 831-834, p. 405, and 856, p. 412). While the struc- 

 ture of the side plates is the same as in Crossometra helius they are very low and 



