322 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



Helix Mitchelliana, DESHAYES in FriR., I. 137, PL XCYII. Figs. 4-7, not 13- 



16. 

 Mesodon Pcrmsylvanica, TRYON, Am. Journ. Conch., III. 44 (1867). 



Western part of Pennsylvania, Ohio, Illinois, Kentucky, Munroe County, 

 Virginia. It thus appears a species of the Interior Region. 



Animal : upper surface of a dull, uniform lead-color, lower surface of the foot 

 lighter ; about twice as long as the transverse diameter of the shell (see B. J. 

 N. H., I. PI. V). 



This species may be readily distinguished from clausa and Mitchelliana by 

 its somewhat triangular aperture, which is more like that of elevata; it is 

 more elevated, has usually 6 whorls, more convex, and with deeper suture than 

 in clausa. In mature shells the inner margin of the peristome, near the colu- 

 mella, has a tooth-like callus, very similar to that often prevailing in forms of 

 exoleta, thyroides, and albolabris. The umbilicus is invariably more or less open 

 in clausa, but closed in Pennsylvania and Mitchelliana. 



Green described this species in 1827, and deposited three specimens of it in 

 the collection of the Philadelphia Academy, where they are still preserved. In 

 1837 another description and an excellent figure were published by Binney in 

 a well-known and widely circulating Journal. It is, therefore, surprising that 

 so many authors and collectors have confounded it with M. clausa, quite a 

 distinct species. Such, however, has been the case, as a reference to the above 

 synonymy will show. It is, however, well known under its correct name 

 by means of the figures published by Binney, Reeve, and Chemnitz, ed. 2. 

 Deshayes is the only one who has figured it under a wrong name. 



Bland has carefully and correctly arranged the synonymy in his valuable 

 "Notes," 1. c. 



Pfeiffer adds doubtfully to the synonymy H. thyroides var. edentula of Beck, 

 Ind. p. 23. 



Jaw very arcuate, of uniform width ; ends blunt ; anterior surface with 1 1 

 stout, crowded ribs, denticulating either margin. 



Lingual membrane (PI. VIII. Fig. E) with 401 40 teeth; 13 perfect 

 laterals. Morse counted 120 rows of 39 1 39 teeth. The outer laterals have 

 the side cusp decidedly developed. 



The upper portions of the genital system (PI. XV. Fig. G) not observed. The 

 penis sac is long and slender, with the vas deferens and retractor muscle enter- 

 ing its apex, and its orifice entering the vagina near its base. The genital 

 bladder is long, stout, cylindrical, with a median contraction ; its duct is hardly 

 distinct from it, with an entrance opposite that of the penis sac. The prostate 

 is very large. 



The animal of this, and many other species, is often overrun with great num- 

 bers of Acari, resembling Acarus limacum of Europe. There appears to be at 

 least two species of them. They are very minute, flesh-colored, and move with 

 great rapidity, often entering and coming out of the respiratory foramen. Their 



