MESODON. 327 



The color of the animal varies in being more or less dark ; but I have never 

 seen an individual which approached the white, pearly, or cream-color, which 

 is so common in the animal of M. albolabris. The eggs are white, one eighth of 

 an inch in diameter, and are laid in the earth as deep as the body of the animal 

 will extend, in clusters of about twenty. 



There is certainly a strong resemblance between many of our species, which, 

 with M. albolabris as their type, form a well-marked division. But as their 

 differences are as constant as their resemblances, it cannot be proper to unite 

 them into one. 



When Dr. Binney published the first description of this shell, in 1837, he 

 adopted, without examination, the name zaleta, which he found applied to it in 

 some cabinets, and which he then supposed had been applied by Mr. Say. 

 Finding no description of it, he subsequently applied the correct name exoleta, 

 originally suggested, no doubt, by the idea that the species is an old or super- 

 annuated form of albolabris. 



Jaw narrow, slightly arcuate, somewhat attenuated towards the ends; an- 

 terior surface with 13 ribs; both margins denticulated. 



Lingual membrane (PI. VIII. Fig. A) with 60 1 60 teeth; 11 perfect 

 laterals, but even the eighth tooth shows a decided modification in form. 



I have already referred to the peculiarity of this species in having sometimes 

 and sometimes wanting side cutting points to the outer lateral teeth, and a 

 bifurcation to the inner cutting point of the marginals (see Proc. Phila. Acad. 

 Nat. Sci., 1875, 243). I here figure teeth from a lingual membrane differing 

 in this respect from that figured by me before (I. c. PI. XI. Fig. 7). The 

 cutting points of the central and first lateral teeth have a lateral bulging 

 which represents the side point. This point appears about the eleventh 

 tooth. 



Fig. a represents an inner marginal tooth from another membrane, agreeing 

 with my former figure in having a simple, not bifid, inner cutting point. 



I am sure of the identity of each individual examined, having verified it by 

 the peculiar genital bladder and penis sac. 



Genitalia figured by Leidy, Vol. I., 1. c. The penis sac is very stout, long, 

 cylindrical, receiving the retractor muscle and vas deferens at its summit; genital 

 bladder subconical, on a short, small duct ; the vas deferens is convoluted as it 

 leaves the prostate. As already stated, these organs are specifically different 

 from those of albolabris, whose shell is so nearly allied to that of exoleta. 



Mesodon Wheatleyi, BLAND. 



Shell imperf orate, depressed, conoid-globose, thin, reddish horn-colored, with 

 numerous rib-like striae, and microscopic granulations with very short hairs ; 

 spire shortly conoid ; suture deeply impressed ; whorls 5|, rather convex, the 

 last rounded, slightly depressed at the aperture, constricted ; base convex, ex- 

 cavated in the umbilical region ; aperture oblique, lunate, with a small parietal 



