174 ILDICA. 



I. NANA Bergh. PI. 69, figs. 50-57. 



Form oval, 2 to 2*5 mill, long, 1/3 to 1/4 mill. high. Color of the 

 back blackish or black, strongly contrasted with the chalk-white 

 shell. Sides of body slightly lighter, but anteriorly coal-black, like 

 the head, the gill grayish ; foot the color of the sides, the viscera 

 showing whitish through it anteriorly. Back even, pretty convex, 

 with the forward slope longest, posteriorly rounded ; anteriorly little 

 narrower with the acute, narrow margins of the body slightly pro- 

 jecting. 



On the hind end of the body, lying slightly to the right or median, 

 is the chalk-white shell (see figs. 52, 53,) placed parallel with the axis 

 of body or inconspicuously oblique, sometimes sunken in a slight 

 depression. 



The shell (figs. 50, 51) is chalk-white, somewhat variable in form, 

 generally long-oval, with pretty parallel side-margins, in front some- 

 what truncate, behind rounded, about '4 mill, long, '16 wide. It is 

 thin but not especially fragile, hardly thinner at the edges, strongly 

 adhering to the integument, level and without any distinct growth- 

 strise. It effervesces violently with acid, and an organic substratum 

 of the form of the shell is left. 



Anteriorly on the back there is no trace of eyes or tentacles. 

 Sides of the body not very low, gradually rising toward the poste- 

 rior. Behind on the right side is the longitudinal gill, which seems 

 to be simple, feathery, and projects slightly beyond the tail. Be- 

 hind it seems to be the anal opening; and before, on the right side ? 

 the minute genital orifice. The head is quite small with perpendic- 

 ular mouth, on each side of which is a quite small tentacle, perhaps 

 with longitudinal furrows. Sole nearly as wide as the back, 

 roundly truncate in front, with fine marginal grooves ; the back end 

 (or short tail) somewhat tapering, rounded, somewhat projecting. 



Radula (figs. 54, 55) not very narrow, with, as it appears, 12 to 13 

 rows of teeth ; further backward there seem to be 18 to 19 developed, 

 and two younger rows ; the entire number, therefore, 32 to 34. The 

 tooth-rows seern, as well as could be judged from the poor condition 

 of the material, to have a lateral on each side of the median tooth. 

 The median tooth (fig. 54) is wide, in the form of a crescent, with 

 an indication of a reduced denticle on each side. Laterals (fig. 55) 

 shaped as in Philine, with the margin smooth, not denticulate. 



The stomach contained, in two individuals examined, 4 strong, 

 erect, yellowish or nearly glassy-clear lunate plates (fig. 57), the 



