260 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



broad, the width at the base being abnost or quite twice the median height to the 

 apex of the angle on the outer edge just above the middle of the base; the proximal 

 process is large and broad; it extends outward for a distance equal to from one- 

 half to two-thirds of the length of the base ; the inner half of its proximal border 

 is concave, the outer half convex ; the edge from the proximal outer corner to the 

 median angle of the outer edge is slightly concave, and to the middle of this edge 

 the covering plate is attached ; between the median angle and the distal corner the 

 outer edge of the plate is deeply concave, having the form of a semicircle; in 

 lateral view the distal outer angle is produced in the form of a hook; its extremity 

 is expanded, this expansion showing in lateral view as a right angle midway 

 between the base and the point. 



The covering plates are very broadly rounded, as broad or as slightly broader 

 than long, with the proximal edge in the form of a broadly obtuse angle, and the 

 inner angle, between the base and the broadly curved side toward the middle of 

 the pinnule, sharper and less rounded than the outer. 



Pachylometra inaiqualis (fig. 1168, pi. 27). 



Chondrometra robusta. — The side and covering plates are essentially as in 

 Perissotnetra lata; the notch between the distal outer process of the side plates 

 and the median notch on the distal border is occupied by a small plate. 



Calyftometra lateralis (figs. 868-871, p. 425). — The side and covering plates do 

 not differ essentially from those of Perissometra patula. In their proximal third 

 the covering plates bear a median keel, placed somewhat diagonally, which is 

 high and triangular, the base of the triangle touching and perpendicular to the 

 proximal edge. 



On the underside of the ventral perisome of the pinnules is a continuous 

 series of large irregular plates forming a continuous broad band. 



Glyptomnetra tuherosa (fig. 816, p. 385). — The side and covering plates are 

 essentially the same as those of Pachylometra distincta, the side plates being very 

 low and broad, twice as broad as high. The side and covering plates are extraordi- 

 narily thin and delicate, composed of a thin calcareous film. The side plates are 

 so low that the proximal process curves around to the proximal end of the base; 

 this process is much finer in texture than the rest of the plate. 



On the underside of the covering plates in their proximal third is a keel, 

 placed somewhat diagonally; from near the end of this keel on the outer side 

 the outer border of the plate curves about in a circle as far as a point on 

 the inner side directly over the outer corner of the proximal process, whence 

 it is continued in a straight line downward to that angle; the portion of 

 the base which lies on the inner side of the keel, or point of attachment, is 

 perfectly straight, meeting the proximal part of the inner border at right 

 angles over the outer corner of the proximal process; the distal and outer 

 portions of the periphery are fringed with fine but prominent spines. Some- 

 times this form of covering plate is not developed, the outer side joining the 

 proximal end of the keel on a level with the inner side instead of the distal end; 

 but in this case the inner basal angle is much less rounded than the outer. On the 

 other hand, the parallel sided portion of the plate, between the median keel and 



