THE NAUTILUS. 



on the under side of stones on a coarse, pebbly beach, and here it 

 still retained its characteristic, long, narrow form. See Proceedings 

 of Boston Society of National History, Vol. 32, No. 2, Nov., 1904. 

 All the illustrations here are of the same magnification. In the 

 fifteen specimens of Acmaea testudinalis collected and studied the 

 radulas were all the same with no observed variation, see PI. II, 

 Fig. 1. All had two central (C C), two lateral (L L), and two 

 outermost teeth (U U), while among the thirty A. alveus were several 

 abnormal radulas, in all of which abnormal cases there were three 

 teeth in the center, PI. II, Fig. 4, instead of two, PI. II, Fig. 2, 

 which is the normal number for alveus. The additional or third 

 tooth (A), see PI. II, Fig. 4, is on the left side and is a narrower 

 tooth than the other two (C C). These abnormal radulas came 

 from three very different individuals : one from a large, blackish, 

 distorted shell, an adult, and one from a small, regular, white 

 specimen of the nepionic age, and so on, all being different. There 

 are several differences between the radulas of testudinalis and of 

 alveus, and these differences were constant throughout the speci- 

 mens examined. 



FIG. 5. 



Fio. 6. 



Fig. 5 (in text). Acmaea testudinalis (Miill). Normal adult rad- 

 ula. Developing teeth of the posterior portion of the same radula as 

 Fig. 1. Lettering the same. 



Fig. 6 (in text). Portion of anterior part of radula of Acmaea te- 

 tudinalis, showing small bases (B), proximal portion of cusp (P C), 



