MESODON. 331 



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

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



Helix thyroidus, Say, Nich. Encycl. (Amer. ed.), 1817, 1818, 1819 ; Journ. Phil. 

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

 33, PI. XIII ; ed. Chenu, Bibl., 3, 22, PI. III. Fig. 3. — Eaton, Zool. Text- 

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

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

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

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

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

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

 mont Mollusca, 159 (1842). — Mrs. 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, 

 PI. 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. PI. LXXVIII. Fig. 3, no descr. 



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



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

 III. 9, PI. 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. Ferussac, Hist, 

 PI. L. a, Fig. 7. 



Mesodon bucculenta, 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 

 narrow, terminated posteriorly in an acute angle. Length equai to twice the 

 breadth of the shell. (See Bost. Journ. N. H., I. PI. VU.) 



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 either margin. 



The lingual membrane (PI. VIII. 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 figured 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. 



