226 INVERTEBRATA OF MASSACHUSETTS. 



at the summit, faintly marked by lines of growth, and separated 

 by a distinct suture ; each of the interior whorls has one or two 

 prominently raised, rounded, revolving hues or keels, and the 

 exterior one has three, one of which issues from the lower junc- 

 tion of the lip and borders the umbilicus ; a second originates 

 from the upper junction of the lip, and circumscribes the whorl; 

 the third midway between this and the suture, thus giving the 

 whorls a prismatic or quadrangular instead of a cylindrical appear- 

 ance ; aperture circular, modified by the keels ; lip simple, sur- 

 rounding the aperture, except a small space between the two lower 

 keels ; umbilicus broad, deep, tunnel-shaped. Height -^V inch, 

 breadth ^^ inch. 



Var. SIMPLEX, without the keels. Professors Benedict and 

 Adams have found this species in Vermont ; and in very many 

 instances it is perfectly simple and cylindrical, individuals occur- 

 ring with every degree of carination, showing that it is still the 

 same species. In this state, the large, tunnel-shaped umbihcus is 

 the best characteristic. 



It is found in most of our small lakes, usually under stones, or 

 sheltered by the deserted shells of some of the fresh-water 

 mussels. 



The shell is usually rendered somewhat opake by an earthy coat- 

 ing, which seems to answer the purpose of an epidermis ; but, when 

 this is removed, the surface is shining and pearly, of an emerald-green 

 color, lighter on the keels. It is one of our most curious shells. 



Valva'ta pupoidea. 



Shell minute^ elevated, chestnut-colored; whorls four or Jive, 

 the last nearly disjoined. 



Figure 155. 



State Coll., No. 65. Soc. Cab., No. 1363. 



Valvkta pupoidea, Gould ; Silliman's Journ., xxxviii. 196. 



Shell small, elongated-ovate, opake, chestnut-colored, when 

 divested of the rough, dirty pigment which usually adheres closely 

 to it ; whorls four or five, minutely wrinkled, the posterior one 

 small and flattened so as to form an obtuse apex ; the others 



