ZONITES. 133 



A species of the Interior Region, traced thus far from the Alleghany Moun- 

 tains to Missouri ; Ohio to Georgia. 



The teeth within the aperture are in general formed of a single prominent 

 lamina, or tooth-like fold ; but sometimes one or both of them are bifid, or even 

 trifid. A second set often, and sometimes a third set, of teeth are seen through 

 the transparent base of the shell, irregularly striated, but generally having 

 equal spaces between each two sets. They are apparent in the youngest as 

 well as in the oldest specimens, and continue to be formed from time to time, 

 so long as the shell increases in size. They probably mark regular periods of 

 growth ; and it may be that these are annual. The growth seems to go on 

 actively for a time, by the addition of new testaceous matter, indicated by the 

 oblique striae, and then alternates with a season of repose, when the teeth and 

 aperture are formed. The teeth appear never to be entirely absorbed and 

 removed, although the aperture, near which they were originally placed, is 

 often advanced very far beyond them. When in motion, the shell lies hori- 

 zontally on the animal's back. 



A curious subject of investigation is the albinism, or entire absence of color- 

 ing matter, in the shells of certain individuals of this and other species. The 

 albinos of this species are of a pure, lively white, while the contained animal is 

 highly colored. Mr. Anthony remarks that about one seventh of all the speci- 

 mens collected by him in the neighborhood of Cincinnati are colorless. As 

 they arc apparently operated upon by the same physical agents which influence 

 the others, it is not easy to conjecture how this singular effect is produced. The 

 animal is sometimes cream-colored throughout; but in such instances the shell 

 is usually colored. 



Animal with head, neck, and eye-peduncles bluish-black or slate-color ; 

 margin and posterior part of foot white. Eye-peduncles very long, tentacles 

 very short ; body narrow and delicate, in length not much exceeding the diam- 

 eter of the shell. I cannot detect any caudal mucus pore, but it is difficult 

 to see any such, even if existing (as I believe it must), on account of the ex- 

 treme transparency of the animal. 



Jaw slightly arcuate, ends attenuated, pointed ; a median, beak-like projec- 

 tion to the cutting edge. 



Lingual membrane as usual in Zonites (PI. III. Fig. Q) with 28 1 28 

 teeth ; 4 laterals. 



The genital system has the dart sac and dart of ligerus. 



Zonites multidentatus, BINNEY. 

 Vol. III. PI. XLVHI. Fig. 3. 



Shell umbilicated, depressed, sub-planulate above, very thin, pellucid ; epi- 

 dermis smooth, shining ; whorls 6, narrow, slightly convex, increasing but 

 slowly in diameter, delicately striated, beneath smoother ; suture impressed ; 



