650 SYSTEMATIC PALEONTOLOGY 



cated, ventral margin approximately horizontal ; umbones orthogyrate or 

 turned a little forward, proximate, often thickened, flattened upon their 

 summits, placed back from the anterior margin a distance of one-third the 

 total latitude; lunule broadly lenticular, sharply denned, the portion in 

 the left valve a trifle broader and more feebly striated by the incrementals 

 than that of the right; escutcheon more sharply defined, broader and a 

 trifle larger in the right valve than in the left ; posterior area outlined by 

 an obtuse ridge passing from the umbones to ths posterior ventral margin ; 

 external surface sculptured with low, concentric ridges close set but 

 irregular in arrangement, suggesting an exaggerated incremental sculp- 

 ture ; a few pronounced resting stages, usually developed toward the ven- 

 tral margin ; radial sculpture manifested only in the sharp denticulations 

 on the inner margins ; hinge plate very heavy, ligament pit a small scoop- 

 shaped affair, extending obliquely backward from directly beneath the tips 

 of the umbones; cardinals two in number in the left valve, three in the 

 right, the anterior cardinal of the right very thin and laminar, and fused 

 at the base with the dorsal margin, the middle cardinal heavy, trigonal, 

 transversely striated; the posterior cardinal laminar largely effaced by 

 the resilium, originating near the base of the anterior cardinal and diverg- 

 ing from it at an angle of approximately 60, cardinals of the left valve 

 much more nearly equal than those of the right, the posterior rather thin, 

 just under the umbones where it forms the anterior margin of the 

 ligament pit, but expanding rapidly toward its ventral extremity; left 

 cardinals striated on their inner faces, separated by a deep trigonal pit 

 for the reception of the right anterior cardinal ; small sulcus near the base 

 of the left posterior cardinal provided for the laminar posterior cardinal 

 of the right valve; no trace of true laterals developed but the posterior 

 dorsal margin of the right valve and the anterior dorsal margin of left valve 

 bevelled to function as laterals and received in grooves in the opposite 

 valves; muscle impressions subequal, placed near the median horizontal, 

 the anterior more deeply excavated than the posterior ; anterior pedal scar 

 very distinct, set under the hinge plate a little dorsal to the anterior 

 adductor; pallia! line entire; inner margins finely crenulated from the 

 ventral extremity of the lunule to the ventral extremity of the escutcheon. 



