122 SYSTEMATIC PALEONTOLOGY 



Cythereis cornuta var. AMERICANA n. var. 



Plate XXXVII, Figs. 29-33. 



Description. — Carapace obliquely subquadrate, the dorsal margin 

 straight and equaling a little more than half the entire length, the ven- 

 tral edge straight in the middle and at the ends, curving more rapidly 

 into the anterior margin, which is most prominent in the lower half, than 

 'into the posterior outline. The latter is the most prominent at a point 

 about a third of the height of the left valve beneath the line of the dorsal 

 edge, and from this point the outline turns anteriorly, at first at a right 

 angle, then with a gentle upward curve on to the well defined post- 

 cardinal angle. The anterocardinal angle is sometimes indistinct and 

 always blunter than the posterior angle. The right valve differs from 

 the left principally in this, that both of the cardinal angles are indis- 

 tinct. Both valves bear a fluted crest, always divided about its mid- 

 length, along the cardinal margin ; and a ventral ridge that begins about 

 the middle of the anterior margin with a gradually coalescing series of 

 spines and continues to rise posteriorly until it terminates in a promi- 

 nent sharp spine, projecting obliquely downward and backward, about 

 one-third of the length of the valve from the posterior extremity. The 

 inner slope of this ridge is fluted like the dorsal crest. From the ter- 

 minal spine the ridge turns upward toward the postcardinal angle, grad- 

 ually growing obsolete before reaching it. Two-thirds of the distance 

 intervening between the two points are marked by prominences, the first 

 being a rather prominent node, the second much more obscure. The com- 

 pressed posterior end terminates in a series of strong spines, six on the 

 left valve and five on the right, while a fringe of smaller spines forms 

 the anteroventral edge. Surface of valves smooth and depressed be- 

 tween the marginal ridges, the valves being on the whole very shallow. 



Length of a relatively short left valve 1.10 mm., greatest height of 

 same 0.60 mm. ; length of a proportionally long valve 1.20 mm., greatest 

 height of same 0.58 mm. Height at posterior and anterior angles of 

 cardinal margin respectfully as nine is to twelve, or four is to five. 



These American specimens are too near the common European Ter- 

 tiary C. cornuta (Eoemer) to be distinguished specifically, but they 

 exhibit sufficient minor differences to justify the subordinate name above 

 proposed. 



