UNICORN SHELLS. 29 



with a spire of four whorls, a rather small aperture, 

 within which are several knobs or teeth. 



The outside is marked by fine spiral grooves, crossed 

 by lines of growth ; and the colors, white and brown, 

 are broken up into little checks, giving it a grayish 

 appearance. I have found many of these moUusks 

 on the rocks, near the upper tide-mark. In such an 

 exposed position they are liable to receive severe 

 knocks, as the the waves come dashing in, and if 

 their shells were light and thin, like some of those 

 which we shall soon have occasion to examine, they 

 would quickly be broken to pieces. Shore shells are 

 usually strong and solid ones. Whenever you exam- 

 ine a shell, please notice such points, and try to find 

 out how it is adapted to its surroundings. In this 

 way, shell gathering becomes something more than a 

 mere pastime, for it brings us face to face with the 

 great questions of life, of design, and of final causes. 

 Along with specimens of Chrysodojuus dirus^ shown 

 in Fig. I, I found on Duxbury Reef a great number 

 of shells quite similar to those of the last species. 

 They were on the moist rocks, under the heavy growth 

 of sea- weed, and were of almost the same color as the 

 stones to which they were clinging. A view of one 

 of these shells is shown in Fig. 10. The 

 name, derived from the angular appear- 

 ance of the whorls, is Afonoceros engona- 

 ticm, Conr., en-go-na^-tum. While the 

 former species resembled a rolled pebble, 

 this one has sharper corners and looks as 

 if it had been less worn. In other respects 

 it is very similar to M. lapilloides^ and 

 Fig. 10. some observers think they are but varieties 

 of the same species. The remarks on a preceding 



