Whitella obliauata.] LAMELLIBRANCHIATA. 565 



of the shell, with two to five rather oblique 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 Jine, where it was 

 supported by an internal ridge in each valve. Two simple adductor impression's, 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. umbonata 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. 13 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 bolh 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 

 case in Whitella. 



Whitella obliquata Ulrich. 



PLATE XL, FIGS. 31 and 32. 



Whitella obliquata TJlkich, 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. Escutcheon well marked, wide, shallowest 



