LAMELLIBRANCHIATA. 565 
Whitella obliquata.] 
of the shell, with two to five rather obilque folds or teeth in front of the beaks. 
Posterior portion of hinge apparently edentulous. Ligament probably both external 
and internal, the latter only along the posterior third of the hinge line, where it was 
' supported by an internal ridge in each valve. Two simple adductor impressions, the 
posterior one very faint; pallial line simple, marginal; interior of shell lined with a 
nacrous film. Surface of shell with fine concentric lines, and sometimes with stronger 
concentric undulations. 
Type: W. obliquata Ulrich. 
No more easily recognized genus of Lamellibranchiata than this is known from 
the Lower Silurian rocks, and of those restricted to that system, none is more 
important in the way of species and distribution. Twelve species, nine of them 
Trenton, the rest from the Hudson River group, are described and figured in this 
work. ‘Two others were described by me in 1890, from the Cincinnati group of Ohio 
as W. wmbonata and W. subovata, while another pair, hindi and plebeia, from the 
Hudson River rocks of Anticosta, were doubtfully referred by Billings to his genus 
Cyrtodonta. With the latter species Billings describes two others as Cyrtodonta ? 
sigmoidea and C. acutumbona (1866, Catal. Sil. Foss. Anticosti, pp. 18 and 49), which 
may turn out to belong to Whitella. The Cypricardites carinata Meek, from Cincin- 
nati, Ohio, also belongs here, while a very large species from the upper beds of the 
same formation remains to be described. Species of this genus have been referred to 
Dolabra McCoy, Cypricardites Conrad, and Cyrtodonta Billings.. McCoy describes his 
genus as containing inequivalve shells in which the hinge is edentulous. Cypricardites 
and Cyrtodonta both have well developed posterior lateral teeth and quite different 
cardinal teeth. In the latter also the shell is thicker and the ligamental area never 
so well developed, nor is the umbonal ridge ever so prominent as is commonly the 
ease in Whitella. . 
WHITELLA oBtiquatTa Ulrich. 
PLATE XL, FIGS. 31 and 32. 
Whitella obliquata ULRICH, 1890. Amer. Geol., vol. vi, p. 177. 
Shell large, oblique, subrhomboidal in outline, produced in the postero-basal 
region, ventricose, with point of greatest convexity above the middle; beaks rather 
small, prominent, slightly incurved, situated nearly one-third of the length of the 
hinge line from its anterior extremity; umbonal ridge well marked, the cardinal 
_ slope concave. Anterior end small, narrowly rounded above, merging gradually into 
the evenly and only moderately convex ventral margin. Posterior end sharply 
curved and produced below, gently convex and sloping forward in the upper half to 
meet the slightly convex, cardinal margin. Hscutcheon well marked, wide, shallowest 
