4 THE NAUTILUS. 



large and small, see PI. II, Fig. 1, that is, the base of the centrals 

 (C C) being large and the cusps small ; whereas in the laterals (L L) 

 the bases are very small and the cusps large. The teeth in both 

 species are set on separate plates, two on each plate. These plates 

 are arranged in two rows, one down each side of the lingual mem- 

 brane. The divisions between these separate plates are not distinct 

 in the developing portion of the radula, but the plates seem to 

 wrinkle with age, and in the center of the radula an elongated oval 

 space is seen between the two. The first two or three rows of teeth 

 in A. testudinalis are slightly blunter than those behind, but by no 

 means so blunt as in A. alveus. What I mean by first teeth are those 

 teeth in the anterior portion of the radula, these teeth are replaced by 

 new ones which come forward from the developing portion. 



Fig. G, drawn by the author, showing plates and proximal portion 

 of cusps in A. testudinalis where the rest is broken of. 



Again the cusps of the teeth of testudinalis are very much more 

 slanting backward or posteriorly than those of alveus, especially the 

 laterals (L L). Lastly, the cusps of testudinalis are minutely granu- 

 lated, while those of alveus are indistinctly striated. This was seen 

 in some cusps broken off their bases, placed on a separate slide and 

 viewed with a one-sixth inch objective. 



In summing this matter up, one might say without much doubt 

 that these two shells were distinct species. Also one may say that 

 both radulas are exceedingly strange. They have no central tooth 

 speaking strictly, as, with the exception of the abnormal radula of 

 A. alveus, all radulas had an even number of teeth. Their deep 

 chestnut color is another curious feature. Also they are very hard 

 radulas to draw as they are raised more than most teeth and the 

 angles and focuses are hard to get. 



My thanks are due to Mr. R. T. Jackson, of Cambridge, and to 

 Mr. J. A. Cushman, of the Boston Society of Natural History, for 

 drawings of the radula. 



EXPLANATION OF PLATE AND FIGURES. 



Fig. 1. Acmaea testudinalis (Mull). Normal adult radula. L, 

 lateral ; C, central ; U, outermost teeth. The cusps are shaded. 



Fig. 2. Acmaea alveus (Conrad). Normal radula. Lettering the 

 same as before. The outermost teeth (U) are so close to the laterals 

 (1) that in this view they seem to be joined. 



