1922] NATURAL SCIENCES OF PHILADELPHIA. 65 



Nevil (1881) a subfossil form of which the operculum was unknown. 

 The peculiar spiral sculpture of the embryonic shell of Pseudo- 

 trochatella, and its habitat, the Island of Mauritius, render its 

 reference to the Helicinidae extremely dubious, at least until the 

 operculum is known. 



In conclusion, it may be stated that the position of the spiral 

 nucleus of the operculum, and the arrangement of the growth- 

 lamellae, furnish excellent characters for the separation of the 

 minor groups. The relative dominance of the horny and calcareous 

 plate, altho somewhat variable, especially with age, are of value, 

 even in the separation of genera. On the other hand, the opercular 

 characters are not of much value in the determination of the 

 relationship between genera, and certainly cannot be used for the 

 division of the Helicinidae into subfamilies. 



Description of Plates III — VII. 



On account of the considerable difference in size, the capituliform complexes 

 are usually shown here with a different magnification than are the centrals and 

 marginals of the same species. In addition, it was found necessary to show the 

 radulae of different species with different degrees of magnification. For these 

 reasons, the centrals and marginals of each species are in a group by themselves, 

 while the capituliform complex is shown separately, usually with less mag- 

 nification. 



In the following descriptions, the magnification is given as 1, 2, 3, 5 or 6 times 

 (diameters). This means that the drawing with the least magnification is taken 

 as a standard, and the enlargement of the others is expressed in proportions to 

 the nearest integer, so as to facilitate their rapid comparison, by the use of a 

 simple fraction. The accurate scale in microns (^ = .001 mm.)is given with each 

 set of figures under the same magnification. Each scale pertains to the figures 

 between it and the next scale above, and to those below in case no other scale is 

 given. All of the drawings were made with the aid of the camera lucida. The 

 magnification under which the drawings were first made is as follows: XI = 248 

 diameters; X2 is a camera lucida reduction from X3; X3 = 705 diameters; X5 

 = 1180 diameters; X6 = 1490 diameters. 



All of the teeth in the series shown are those from the right side. As the central 

 groups are all arranged in the same order, with the R-central at the left hand of 

 the sheet, followed by the A, B and C centrals in order named, they are not 

 specially numbered. The marginals are numbered as they occur in the transverse 

 row, from the center out. The figures themselves are numbered separately at 

 the edge of the plate. All of the teeth, including the tips of the marginals, are 

 oriented into the most common position, although this is necessarily often only 

 approximate. 

 Plate III. — Hendersonia, Oligyra, Lucidella and Helicina. Centrals and Marginals. 



Fig. 1 — Hendersonia occulta. Centrals and tips of lrst, 8th and 13th marginals 



(x5). 

 Fig. 2 — Oligyra orbiculata. Centrals (x5). Compare fig. 1 for marginals. 

 Fig. 3 — Oligyra (Succincta) flavida streoeli. Centrals and tips of 1st, 5th, 



and 12th marginals (x5). 

 Fig. 4 — Lucidella aureola. Centrals and tips of 1st, 6th and 40th marginals 



(x6) (A-central shown below in tilted position). 



