THE NAUTILUS. 3 



where the rest has been broken off, and plates (P) on lingual mem- 

 brane (L M). U, outermost; L, lateral; C, central teeth. R, 

 turned oyer part of plate. B, base. 



Tbere is a distinct dip in the proximal portion of the cusp of testu- 

 dinalis in the laterals : this does not appear in alveus. In testudinalis 

 the bases project from the proximal portion of the cusp at an obtuse 

 angle, while in alveus the proximal portion of the cusp is the same 

 size as the adjoining portion of the base. In testudinalis, PI. II, 

 Fig. 1, the apex of the cusp is just inside of the letter (L), from 

 here it slopes down to the base ; while in PI. II, Fig. 2, alveus, the 

 apex of the cusp runs the whole length of the foremost line border- 

 ing the shaded portion. In testudinalis the lateral teeth form a 

 letter v, while in alveus they form an inverted v, thus a. In alveus 

 the laterals are less underneath the centrals than in testudinalis. 

 The teeth of testudinalis are sharply rounded at the apex of the cusp, 

 while those of alveus are nearly square, PI. II, Fig. 2, and the 

 younger, undeveloped teeth of the radula of an adult shell of alveus 

 are perhaps the squarest of all, PI. II, Fig. 4. This tigure is of the 

 developing teeth of the posterior portion of the same radula as PI. II, 

 Fig. 2. That is that portion hidden within the throat which will 

 later come forward to take the place of worn-out teeth. I thought 

 of the various causes of these differences, but it could not be in- 

 dividual variation, because the differences were constant throughout 

 the forty-five specimens examined. It could not be because the 

 teeth were worn square, as the squarest of them in alveus are in the 

 early, developing part of the radula and before they had been used 

 at all, see PI. II, Fig. 3. There were five less important differences 

 between the radulas of testudinalis and of alveus, these are : First the 

 outermost tooth (U) is, in alveus, larger in proportion to the lateral 

 tooth (L) than the outermost tooth of testudinalis is to its lateral 

 tooth (L). In both species this little outside tooth (U) is nearly at 

 right angles to the next tooth and the cusp of U faces in center of 

 the lingual membrane (L), also it is very close to it, and at first 

 glance appears to be the same tooth, only being a cusp of the larger 

 denticle, but when viewed in a sideways, sectional view it is seen to 

 be distinct. In the species testudinalis the tooth (U) cannot take 

 much part in cutting food as it so much underlies the larger tooth (L), 

 but in alveus, where it is more outstretched, it may do a good deal 

 of work. Secondly, in A. testudinalis the cusps and bases alternate 



