n 



30 



PROCEEDINGS OF THE ACADEMY OF [Vol. LXXIV 



The operculum consists of two quite distinct layers, an inner 

 horny layer, which is often lacking in dried specimens, and an 

 outer, calcareous one. The horny layer shows the growth-lamellae 

 and the spiral nucleus much less distinctly than does the calcareous 

 portion. The relative development of these two layers gives an 

 additional character, little used by Wagner. 



The radula of the family has been variously described and figured 

 but, in most cases, neither figures or descriptions are very usable. 

 Under low magnification, it appears to consist of a narrow, trans- 

 parent trough, with a broader, rounded ridge on each side. It is 

 long and narrow, and in most of the species observed by me, consists 

 of about 100 transverse rows, although the extremes show from 

 80 to 171. This character seems to be mainly dependent on size, 

 and is of little value in the determination of relationships. 



Under higher magnification, five groups of teeth can be made 

 out: a central group; and a lateral or capituliform complex, and 

 a marginal group on each side. The central group consists typically 

 of seven, very small, almost colorless teeth (Plate III) : the unpaired 

 or rhachidian tooth and three paired centrals on each side. For 

 brevity, in the following descriptions the unpaired central is 

 termed the R-central (at the left hand in the figures), while the 

 paired centrals are called the A-central, the B-central and the 

 C-central, in order from the inside out (left to right in figures). 

 The R-central is usually a thin and transparent plate, which is 

 bent upwards at its distal end so as to form a broad scraping edge, 

 or cusp. The paired centrals are variously shaped and cusped, 

 but the A-central and B-central primitively appear more or less 

 boot-shaped, while the C-central is usually more or less lanceolate 

 in outline 



The lateral or capituliform complex consists of two structures, 

 which are so closely united, as at first to give the impression of a 

 -mull', large and complicated lateral (the capituliform lateral of 

 I' ischer, (1885.)) Closer examination shows this complex to consist 

 <>t two pieces: a comb-shaped (or T-shaped in Vianinae) lateral 

 and ;i variously modified, outer accessory plate. The comb-lateral, 

 as n may be termed I Plate IV, fig. 17, left), is more or less trape- 

 zoidal or spoon-shaped as viewed from the central side, and the 

 upper edge is thickened and reflected (Plate IV, fig. 17a). This' 

 upper edge i^- so heavy as to appear horn-colored, even under 

 agnification, and bears the large cusps. The outer, handle 



