148 Trans. Acad. Sci. of St. Louis. 
broadly rounded, very near the suture, the fasciolar surface 
below the suture broad and feebly concave, the periphery ob- 
tuse and not very prominent, and the suture simple, without 
subjacent elevated collar. The surface is rendered somewhat 
rough by relatively fine, close-set and irregular spiral lines, 
and there is no longitudinal sculpture except lines of growth. 
The species are large and ponderous, and include Surcula 
carpenteriana and tryoni of Gabb, from the coast of Califor- 
nia. Megasurcula is a widely isolated and strongly character- 
ized genus, belonging exclusively to the living fauna of the 
Pacific coast of North America as far as known at present. 
It is, at the same time, a rather direct descendant of the ex- 
tinct Bathytoma, but the species are of far larger size, carpen- 
teriana being probably the largest or most ponderous Pleuro- 
tomid known. The embryo, which is conoidal and multispiral 
in Bathytoma, has gradually lost some of its whorls, as shown 
in Megasurcula, which of itself would not be a generic char- 
acter, but there is in Bathytoma a broad constriction of the 
body whorl below the convexity, forming a short stout beak, 
which is wholly unobservable in Megasurcula, and the aper- 
ture is much more capacious in the latter, with the anal sinus 
much larger and different in form and position. 
Asthenotoma Harr. et Burr. 
Oligotoma Bell. (nom. praeocc.). 
This genus, entirely represented by extinct species of slen- 
der form and elevated, evenly and gradually acuminate spire, 
conspicuous development of the spiral lyrae and short aper- 
ture, should evidently be considered with the preceding gen- 
era and especially with Zrypanotoma and allies, but it is 
somewhat of an annectant form, as the American species at 
least have true ribbing on the nepionic whorls which be- 
comes completely lost on the larger volutions of the shell. 
It is therefore one of those puzzling exceptions which render 
an arrangement of the genera in a dichotomous table so diffi- 
cult and unsatisfactory. The embryo in the type, Pl. bas- 
teroti Desm., of the European Miocene, is said by Cossmann 
