HELIX-ISOMERIA. 155 



versely dilated seen from above or below, convex beneath, and slightly 

 or scarcely at all angled at the periphery ; deflexed a short distance 

 to the aperture, scarcely constricted behind the peristome except on 

 the base ; aperture quite oblique, truncate-oval, brownish or livid in- 

 side ; peristome light brown, narrowly reflexed all around, upper and 

 outer margins regularly curved, and developing usually a small node 

 or tubercle where they meet ; basal margin straightened, becoming 

 more widely reflexed toward the axis, and bearing inside two 

 tubercles, near to each other ; columellar margin dilated, expanded 

 part-way over the umbilicus. 



Alt. 12, diam. maj. 22, min. 18? mill. (Specimen.) 

 Alt. 12, diam. maj. 19*, min. 16i.mil!. (Specimen.) 



Alt. 17, diam. maj. 29, min. 23 mill. (Hidalgo.) 



Ecuador. 



H. bituberculata PFEIFFER, Proc. Zool. Soc. Lond. 1852, p. 153 ; 

 Kiister's Couchyl. Cab., p. 369, t. 139, f. 14, 15 ; Monographia, in, p. 

 242. HIDALGO, Viage al Pacifico, Mol., p. 14. Dentellaria 

 bituberculata MILLER, Malak. Blatter 1878, p. 165. H. bituberculata 

 DOHRN, Jahrbiicher d. Deutschen Malak. Gesell. 1879, p. 186.- 

 Dentellaria tridentula KONRAD MILLER in Malak. Bliitter 1878, p. 

 165, t. 7, f. 5. Dentellaria latidentata MILLER, loc. cit., p. 166, t. 

 8, f. 1. Helix bourcieri REEVE, Conch. Icon., f. 545 (not bourcieri 

 Pfeiffer). 



This small species is often almost smooth, but generally granules 

 are visible on the whorls under a lens. The two basal tubercular 

 teeth, partly open umbilicus, narrow peristome, etc., separate it from 

 allied forms. 



Pfeiffer's description of this species was read under date of Decem- 

 ber 14, 1852, before the Zoological Society, while Reeve's description 

 was published in May of the same year. In this instance, as that of 

 H. atrata, I prefer not to disturb the well-known nomenclature. 

 Pfeiffer's mss. names were in the Cumingian collection before Reeve 

 published. 



Dr. Dohrn is doubtless correct in uniting the forms described by 

 Miller as latidentata and tridentula to bituberculata; considerable 

 variation in width of the umbilicus and size and development of the 

 lip-teeth is shown in the specimens before me. 



Var. TRIDENTULA Miller. PL 43, figs. 43, 44, 45. 



Im perforate chestnut-colored, sometimes with oblique yellow 

 streaks ; right margin of lip with a small tooth ; basal margin with 



