MESODON. 331 



tially covered by the reflected peristome where it unites with the base of the 

 shell. Greater diameter 22, lesser 19^ mill. ; height, 13 mill. 



HcU:c thyroidus, Say, Nich. Encycl. (Amcr. cd.), 1817, 1818, 1819 ; Joura. Thil. 

 Acad., 1. 123 (1817) ; American Coiichology (1831), No. 2, PI. XIII ; ed. Binney, 

 33, ri. XIII ; ed. Chknu, Bibl., 3, 22, PI. III. Fig. 3. — Eaton, Zobl. Text- 

 Book, 193 (1826). — FitiiussAC, Hist., PL XLIX. a, Fig. 4 ; PL L. a. Fig. 6? — 

 Deshayes, EncycL Meth., II. 230 (1830) ; in Lam. An. sans Vert., VIIL 114 ; 

 ed. 3, III. 309; in Fi5r., I. 209. — Binney, Best. Journ. Nat. Hist., I. 488, 

 PL XVIII. (1837) ; Terr. MolL, II. 129, PL XL — Leidy, T. M. U. S., I. 257, 

 PL XL Figs. 7-9 (1851), anat. — DeKay, N. Y. MolL, 29, PL II. Fig. 8. — 

 Gould, Invertebrata, 171, Fig. 108 (1841) ; ed. 2, 425 (1870). — Adams, Ver- 

 mont Mollusca, 159 (1842). — Miis. Gray, Fig. Moll. An., PL CCXCI. Fig. 6, 

 from Bost. Journ., no descr. 



Helix thyroides, Pfeiffer, Mon. Hel. Viv., I. 345 ; in Chemnitz, ed. 2, I. 331, 

 PL LVIII. Figs. 8, 9 (1850). — Reeve, Con. Icon., No. 677. — W. G. Binney, 

 Terr. MolL, IV. 53 ; L. & Fr.-W. Sh., I. 147, Fig. 251 (1869). — Morse, Amer. 

 Nat., I. 98, Fig. 3 (1867). 



Anchistoma thyroides, H. & A. Adams, Gen. PL LXXVIIL Fig. 3, no descr. 



Mcsodon thyroides, Tryon, Am. Journ. Conch., III. 41 (1867). 



Helix hucculenta, Gould, Proc. Bost. Soc. Nat. Hist., III. 40 (1848) ; Terr. MolL 

 III. 9, PL XL a. — Pfeiffer, Mon. HeL Viv., III. 271'; IV. 323. — W. G. 

 Binney, Terr. MolL, IV. 54 ; L. & Fr.-W. Sh., I. 148, Fig. 254 (1869). 



Helix thyroides, /3, Pfeiffer, Mon. Hel. Viv., I. 345. — Var. Fii;russac, Hist., 

 PL L. a. Fig. 7. 



Mesodon hitcculenta, Tryon, Am. Journ. Conch., III. 41 (1867). 



Animal : color a dirty yellowish-white, with a grayish hue in some indi- 

 viduals, eye-peduncles darker, eyes black, base of foot dirty white ; foot rather 

 nan'ow, terminated posteriorly in an acute angle. Length equal to twice the 

 breadth of the shell. (See Bost. Journ. N. H., L PL VII.) 



A Post-pleiocene species now found over all the Eastern Province. The 

 variation in size of the species is great. The smaller form (from near Phila- 

 delphia) is often found imperforate and toothless. 



A reversed specimen was found by me at Graniteville, South Carolina. 



Jaw long, narrow, slightly arcuate, with 13 stout ribs on both anterior and 

 posterior surface, denticulating cither margin. 



The lingual membrane (PL VIIL Fig. S) has 60—1—60 teeth, with 11 lat- 

 erals. This species is peculiar in having extremely long cutting points to the 

 single cusp of its marginal teeth : the very extreme marginals have this cutting 

 point bifid, and also have a small side cutting point. A similar dentition is 

 found in clausa and Wheatleyi. 



The genital system is figdred by Leidy (1. c). The penis sac is short, stout, 

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

 genital bladder is small, elongated, bluntly tapering at its apex, on a short, 

 narrow duct ; the oviduct is greatly convoluted. 



