550 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



radula resembles that of Potamolithus in the important character of having 

 many minute cusps on the marginal teeth, more than double the number 

 of those on the admedian tooth. It differs from Potamolithus by having 

 a single basal cusp or denticle on each side of the central tooth, Potamo- 

 lithus having two or more. I have found the number of basal cusps so 

 variable in many genera that I do not attach much importance to this 

 character. Of all known genera, Petterdiana is, in my opinion, the most 

 closely related to Potamolittms. 



Marginal teeth with a small number of cusps characterize the subfamily 

 IMhoglyphina. The Austral genera Potamolithus and Petterdiana cannot 

 be included in this subfamily. They may for the present be placed in the 

 Amnicolince. 



LITTORIDINA Souleyet. 



Littoridina Souleyet, Voyage autour du Monde execute pendant les annees 

 1836 et 1837 sur l a corvette La Bonite, Zoologie, II, p. 565 (1852). 

 Monotypic : type L. gaudichaudii Soul. 



"IMtorinida Eydoux et Souleyet" Stimpson, Researches upon the Hydro- 

 biinae, etc., 1865, p. 43; Fischer, Manuel de Conchyliologie, 1885, 



P- 730. 



Paludestrina in part, d'Orbigny, 1839; Stimpson, 1865. 

 Heleobia Stimpson, Researches upon the Hydrobiinas, etc., 1865, p. 47. 



Monotypic : type P. culminea. 

 Hydrobia and Pahidina of some authors. 



The shell is small, very narrowly rimate, acutely ovate, thin, smooth 

 or rarely striated spirally, of olive or pale corneous color ; whorls usually 



FIG. 7. 



l, ..Pen. 



Littondina guadichaudii, anterior part of the body, the pallial cavity opened and spread to the 

 left, a, anus ; Pen., penis. (After Souleyet.) 



