NO. 1139. DEEP-WATER MOLLUSC A VERRILL AND HUSH. 807 



somewhat upturned. The entire body of the shell is covered with 

 numerous narrow, elevated, radiating- ribs, separated by much wider 

 concave interspaces, some of the widest of which have a small second- 

 ary rib in the center toward the margin; the ribs increase in elevation 

 and strength posteriorly, toward the base of the rostrum, but never 

 become broad 5 for a short distance on the base of the rostrum the ribs 

 are nearly obsolete but become prominent again on its dorsal and ter- 

 minal portions; this part is also crossed by irregular raised lines of 

 growth which cross the ribs obliquely; the inner surface is covered 

 with rounded grooves corresponding to the external ribs, separated 

 by convex ribs of about the same width; these become obsolete ante- 

 riorly and posteriorly. The hinge-margin in the left valve is only a 

 little thickened and slightly excurved, the cartilage-plate is central, 

 stout, regularly ovate in form, with a thickened inner margin; in the 

 right valve there is a prominent, rather stout, elongated posterior 

 tooth, the anterior end of which joins closely the cartilage-plate, leav- 

 ing scarcely any notch between ; the highest part of the tooth is near 

 the middle, the slope, however, is a little steeper anteriorly; a deep 

 groove separates the tooth from the thin, slightly excurved dorsal mar- 

 gin; anteriorly the margin is but slightly thickened, and shows a very 

 narrow, beveled edge externally for the attachment of the thin liga- 

 ment; a similar but more distinct ligamental groove extends from the 

 beak to the base of the rostrum; there is a short, rather stout, rib-like 

 clavicle or buttress running from beneath the middle of the tooth 

 obliquely backward and downward in the direction of the base of the 

 rostrum ; a less prominent buttress is also present in the left valve. 



Length of one of the largest specimens, 25 mm. ; height, 15 mm.; 

 thickness, 14 mm.; from beak to end of rostrum, 13 mm.; to autero- 

 ventral margin, 12 mm. One badly broken valve is considerably larger 

 than this. There are also two young live specimens which measure 

 about G mm. in length and 3.5 mm. in height. Their form is somewhat 

 narrower and longer than in the adult, and the rostrum appears rather 

 longer and narrower; the postero-dorsal margin is nearly straight; the 

 ventral margin is decidedly concave at the base of the rostrum; the 

 shells are very thin, somewhat transparent and glossy, and have about 

 twenty six sharply denned, considerably elevated, nearly equal, nar- 

 row ribs on the body of the shell, separated by much wider spaces; 

 the edge of the left valve overlaps that of the right, especially along 

 the base of the rostrum. 



In general appearance this species greatly resembles C. multicostata 

 Verrill and Smith. It differs, however, in having a regularly more ovate 

 form with the anterior region somewhat narrower and more prolonged 

 and the postero- ventral margin less incurved at the base of the ros- 

 trum, so that the latter is broader and less differentiated. The exter- 

 nal costa? differ in being narrow and sharp, separated by broad concave 

 interspaces, and of nearly uniform size, there being no marked contrast 

 between those on the anterior and posterior portions of the shell, 



