MESODON. 319 



gray to blackish ; upper part of head and neck slightly brownish : extremities 

 of eye-peduncles smoky; eyes black. Eye-peduncles more than 12 mill, in 

 length when fully extended, sender and cylindrical. Foot with a slightly ex- 

 panded margin terminating posteriorly in an acute angle. Glandular tuber- 

 cles very distinct and prominent, on the back arranged longitudinally, on the 

 eye-peduncles long and narrow. Extreme length, 62 mill. (See Vol. III. PI. II.) 



The animal deposits about fifty eggs at each laying, which is repeated one or 

 more times during the season. The eggs are three sixteenths of an inch in 

 their greatest diameter, and covered with minute points. The last laying is 

 often delayed to so late a period of the year that the earth is covered with snow 

 before they are hatched. The development of the embryo is then suspended 

 until the next spring. When newly excluded from the egg the shell consists 

 of one whorl and a half, the length of its column or axis being about one eighth 

 of an inch, and its breadth somewhat less. No umbilicus is then discernible. 

 I have not been able to determine how much time i: I to complete its 



growth, but I am induced to believe that the peristome, the evidence of 

 maturity, is added in the second year. 



The jaw is arcuate, of uniform breadth throughout ; ends blunt, smooth on 

 their anterior surface, the balance of the jaw with 10 stout ribs, denticulating 

 either margin. 



Outer laterals of the lingual membrane have distinct side cusps as well as 

 cutting points. Teeth 44— 1—44, with about 12 laterals. (PL VIII. Fig. K.) 



Genitalia, as well as complete anatomy, figured by Leidy, 1. c. The penis 

 sac is stout, rather short, cylindrical, with a median prepuce (/>) ; it receives 

 the vas deferens at its summit; the retractor muscle is inserted on the vas 

 deferens near its junction with the penis sac; the genital bladder is long, stout, 

 blunt at its summit, its duct is very narrow at its entrance into the bladder for 

 a short portion of its course, then becomes suddenly expanded into very much 

 the shape and still greater size of the bladder. This peculiar arrangement of 

 the genital bladder and its duct forms a good specific character, distinguishing 

 albolahris from exoleta and other species. I have found its characters constant 

 in the numerous individuals I have examined. As it is wanting in the figure 

 given by Semper (Phil. Archip., PI. XIV. Fig. 16), I am inclined to doubt 

 the identity of his specimen. Lehmann (Mai. Blatt,, XL PI. I. Fig. 1, 1864) 

 no doubt drew his figure from a true albolabris. 



The figure of the jaw given by Leidy represents it imbedded in the tissues 

 of the head above. 



Mesodon divesta, Gould. 

 Vol. III. PI. XIII. a, Fig. 2. 



Shell imperforate, depressed, somewhat discoidal, of medium thickness and 

 a dingy horn-color, sculptured with coarse oblique furrows ; spire slightly con- 

 vex, whorls about 6, a little convex, and separated by a well-impressed suture ; 



