262 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



the inner edge, may be considerably elongated, in some cases occupying about one- 

 third of the entire length of the plate. In the covering plates an area in the shape 

 of a right-angled triangle extending from the edge adjacent to the proximal 

 extension of the side plate to a point as far as the length of this side on the inner 

 edge and thence to the proximal end of the median keel is of much finer texture than 

 the rest of the plate. 



Crinometra insculpta. — Both side and covering plates are relatively small, 

 coarse in texture, with unfinished edges. The side plates are about as high as 

 broad at the base ; the inner half of the proximal side is excavated in the form of 

 almost a semicircle; the outer half is straight and perpendicular to the base; the 

 distal side is concave, the maximum depth of the concavity, which is not very 

 deep, being just below the center; the median portion of the outer edge is pro- 

 duced into an obtuse angle ; the border between this angle and the proximal outer 

 angle is straight or very slightly concave; between this median angle and the 

 outer distal angle is a more or less deep concavity, so that the outer distal 

 angle, which is more or less produced, takes more or less the form of a horn 

 curved distally and outwardly. The inner side of the side plates shows two 

 ridges perpendicular to the base, one near the proximal, the other near the 

 distal border; near the base these suddenly diverge; the distal runs into the distal 

 horn-like process. The base of the side plates is parallel with the edge of the 

 pinnulars; the distal third of the distal edge, embracing the distal process, is 

 approximately at right angles with the proximal two thirds, the two being connected 

 by a broad curve; a line bisecting this angle would be perpendicular to the longi- 

 tudinal axis of the pinnule. 



The broadly oval covering plates, which are broadest between the base and the 

 middle, are attached halfway between the middle of the side plates and their 

 proximal outer angles. On their underside they have a narrow ridge in their 

 proximal third or half which is in contact with the proximal ridge on the side 

 plates. 



M onachometra fragilis (figs. 824^830, p. 399). — The side plates are in close 

 lateral apposition with straight uiner, proximal, and distal edges, higher than 

 broad at the base, the proximal border slightly longer than the distal ; the median 

 portion of the outer edge is produced into a slightly obtuse angle, which reaches 

 the same height as the proximal outer corner ; the border between the median angle 

 and the proximal outer corner is slightly concave ; the border between the median 

 angle and the distal outer corner is deeply concave, so that the latter is in the form 

 of an acute angle; just below the distal outer process and slightly farther from 

 the excavated outer edge than from the distal edge is a circular hole of rather small 

 size, allowing for the passage of the sacculus to the exterior ; the inner portion of 

 the side plates is plane and parallel with the edge of the pinnulars ; the outer border 

 is a broadly rounded right-angled triangle, with the proximal limb, from the 

 proximal outer angle to the notch between the distal outer process and the median 

 angle, much longer than the distal ; both are inclined to the line of the edge of the 

 pinnulars at an angle of about 45°. The interior surface of the side plates bears 

 two prominent parallel narrow ridges of unusual height ; one of these runs fairly 



