NO. 1139. DEEP-WATEU MOLLUSCAVETtRTLL AND BUSH. 815 



horizontal, slightly convex or nearly straight; the ligament is very 

 prominent behind the beak, extending backward in a conspicuous 

 groove nearly to the posterior end, and terminates anteriorly in a deep 

 narrow groove directly under the beak. In the left valve the inner 

 edge of the posterior hinge-margin is somewhat sinuous; just behind 

 the beaks, opposite the most prominent part of the nmbos, it is thick- 

 ened and somewhat revolute, decreasing both in thickness and eleva- 

 tion to a shallow indentation of the margin; back of this, it increases 

 regularly in thickness and prominence and is again revolute along the 

 posterior part of the ligamental furrow. There is no central tooth nor 

 any distinct resilium. In the right valve the posterior hinge-margin is 

 even more thickened and revolute just back of the beaks, and the in- 

 dented, thinner portion, at the end of the prominent part of the liga- 

 ment, is more marked. The ligamental groove is consequently less 

 conspicuous, being partially concealed by the revolute margin. The 

 antero-dorsal margin is compressed and projects strongly upward, 

 rising distinctly above the umbos in a side view and is more convex 

 than in the left valve. There is also a slight elevation within the dor- 

 sal margin directly below the beaks, which might be considered the 

 rudiments of a tooth. 



The largest specimen, when perfect, would be about 15 mm. long. 



Two very much broken valves, station 2229, N. lat. 37 38' 40", W. long. 

 73 10' 30", in 1,423 fathoms, 1884. 



This species somewhat resembles Cetocltonca nitida (Verrill) 1 Dall. 2 

 It is however more oblong, with the urnbos much smaller and less 

 prominent and the beaks less spiral and nearer together. The granu- 

 lation of the surface is somewhat stronger and more generally distrib- 

 uted. The ligamental groove is longer, deeper, and the ligament itself 

 is more prominent behind the beaks. The angnlation of the hinge-mar- 

 gin of the left valve in a horizontal plane is a peculiar feature not found 

 in the other related species and indicates that the valves are decidedly 

 unlike in form, but the right valve is too much broken to show the 

 anterior margin. 



CETOMYA species. 



A broken left valve (No. 52013) from station 2481, N". lat. 44 T 30", 

 W. long. 57 1G' 45", in 110 fathoms, resembles Poromya (Getomya) 

 elongata Dall, from the West Indies and Barbados, in 100 to 119 fath- 

 oms. It is, however, too incomplete for determination without direct 

 comparison with authentic specimens. 



It is larger and more strongly truncate posteriorly than Poromya 

 granitlnld, (Nyst) Forbes and Hanley, and the granules are coarser and 

 not so numerous. It differs, moreover, very strongly in the hinge 

 characters, for the hinge-plate is much thinner and the large tooth in 

 the left valve is wanting in our species. 



1 TJiracia iiilida Verrill, Trans. Conn. Acad., VI, p. 221, pi. xxxn, fig. 22, 1884. 



2 Cctochonca nitida Dall, Bull. Mus. Comp. Zool., XII, p. 281, 1886. 



