DE>fTALIUM-COMPRESSIDENS. 123 



Var. DiDYMUM Watson. PI. 7, fig. 20. 



Shell extremely attenuated, very slightly curved, a. little flattened 

 laterally, and that chiefly towards the convex curve, so that the 

 form is slightly trigonal, porcellanous, pure white, brilliant. Sculp- 

 ture: very fine, irregular scratches run around the shell, the sur- 

 face of which is not perfectly uniform ; there are very faint indica- 

 tions of longitudinal texture, and there is in the substance of the 

 shell a certain transverse flocculence. Towards the mouth the shell 

 is extremely thin as usual ; but towards the apex it becomes thick 

 from the smallness of the bore, which lies not in the center, but 

 nearer the convex curve of the shell. Length r08, breadth 0"G, at 

 apex 0*04 inch. The measurement is taken from the largest of six 

 fragments, none of which preserve the apex of the shell (Watson). 



This may be a variety, distinguished by the slightly trigonal or 

 ovate section, typical ensiculus being regularly, symmetrically ellip- 

 tical at the aperture, as shown in fig. 7. 



Subgenus Compressidens Pilsbry & Sharp, 1897. 



Shell small, decidedly tapering, conspicuously compressed between 

 the convex and concave sides; weakly sculptured, nearly smooth; 

 anal orifice simple, without slit or notch. Type D. pressum Sharp 

 & Pilsbry. 



The few species of this well-defined group inhabit widely separ- 

 ated areas. D. press^im and ophiudon are Antillean, D. brevicornu 

 west American, and D. platyceras Australian. The occidental forms 

 live in deep water. Specimens of all of them have been examined 

 in the preparation of the following pages. The figures on plate 22 

 are from camera lucida drawings, representing concave and lateral 

 aspects of each species. 



An Oligocene member of the group is D. prcBCursor Pilsbry & 

 Sharp from San Domingo. It was probably an ancestor of the re- 

 cent American species. 



Key to Species. 



I. Shell but slightly curved, strongly compressed. 



a. Length about 9*5 mill., about 4 times the greatest diam- 

 eter; Panamic, brevicornu, -p. 125. 



a'. Length 11-13 mill., about 6 to 7* times the greatest 

 diameter; Antillean, pressum, \>. \2\. 



