120 Dentition of Puhnonate Mollushs. 



plate-like sections, of the character found in the jaws of Oylindrella, Macro- 

 ceramus, and many species of Bulimulus. No appearance of triangular upper 

 median plates, however, as in Oylindrella, though the two specimens ex- 

 amined by me are not perfect at that part. Both margins serrated by the 

 extremities of the ribs. The jaw is quite membranous. 



Lingual membrane (PI. XIII, fig. E). Centrals subquadrate with a very 

 large, stout, short, pointed cusp, the side cusps obsolete. Laterals larger 

 and more narrow than the centrals, bicuspid, the inner cusp greatly pro- 

 duced, broad and quite squarely terminating. The base of attachment of 

 the laterals is cut away on the inner side, leaving a large outer lateral ex- 

 pansion, bringing to mind the much less developed one of Succinea. 

 Marginal teeth quadrate, gradually becoming modified from the laterals, . 

 the cusps finally passing off into simple, obtuse papillae, the inner one the 

 larger. 



Bulimulus. 



The species of this genus may be grouped by their lingual 

 dentition, independently of the character of the shell, into 



(a) those having the type of teeth usual in the Heliciclce, and 



(b) those haviug the peculiar type of dentition figured on PL 

 XII, fig. G (see B. primularis) , and (c) those having the denti- 

 tion of B. Lobbi. (PL XIV, fig. E.) The former two types are 

 found in several of the subgenera noticed below. 



The jaw of Bulimulus is usually thin, with delicate, separated 

 ribs as in Oylindrella, their ends serrating either margin ; the 

 ribs at the upper centre of the jaw often run obliquely and ter- 

 minate before reaching the lower margin. 



DrymtBus altoperuvianus, Eve. Between Balsas and Caja- 

 marca, Peru. Prof. Orton. 



Jaw with 31 ribs, delicate, separated, as in Cylindrella. 



Lingual membrane (PI. XIV, fig. F) very peculiar, resembling that of 

 B. Lobbi, described below, excepting that the marginal teeth are of same 

 type as the laterals, with still more produced cutting point; one in profile 

 is shown in the left-hand figure. 



Drymoeus Vincentinus, Pfr., var. ? Tobago. 



Jaw as usual in Bulimulus, thin, transparent, with numerous delicate, 

 separated, narrow ribs. 



Lingual membrane as in Bulimulus laticincius. (See below.) 



Drymmus Knorri, Pfr. Porto Cabello, Venezuela. Mr. Robert 

 Swift. 



