ii8 Studies in Geology, No. 3 



bonal slope rounded anteriorly and mesially, the anterior 

 surface being modelled outward somewhat to meet the 

 slightly produced anterior margin, and the central surface 

 evenly round. Posterior slope not entirely known, but 

 there is apparently a keel simJlar to that in the forms related 

 to this. Ribs, 30-32 in number, sub-square in cross-section, 

 equal in width to the interspaces over entire surface, except- 

 ing one or two interspaces just forward of the posterior 

 keel, which are slightly broader, showing growth-lines, with 

 irregular tendency to form knobs, and with pronounced im- 

 brication of growth-layers near the ventral margin. Area 

 large, long and broad, with 6 to 7 simple wave-shaped 

 furrows which form angles obtuse near the hinge and more 

 acute toward the beaks. Hinge-plate about 5 mm. or more 

 wide anteriorly, 1.5 mm. wide mesially, lost posteriorly; 

 teeth fine below beaks, although coarser than in A. toroensis, 

 increasing in length and coarseness distally. There is a 

 growth on of semi-elliptical cross-section, viewed laterally, 

 on the hinge, which may represent the septum of Grzy- 

 bowski's figured specimen, but it is too poorly defined to sub- 

 stantiate the characteristic presence of the feature. Length 

 estimated at 58-60 mm.; height, 54.5 mm.; (semi) diameter, 

 28.5 mm. 



This species is most closely similar to A. toroensis, of the 

 Zorritos fauna. It dififers from that form in its more closely 

 proximate, fuller, and more highly incurved beaks, its 

 heavier hinge-line, with generally coarser teeth, and its 

 simply and evenly furrowed area : the area of toroensis is 

 furrowed irregularly in most specimens. The septum noted 

 by Grzybowski in his description of Area septifera is not 

 typically present, and although there is in the form here 

 described a perceptible semi-elliptical thickening of the hinge- 

 plate beginning about 10 mm. behind the vertical line of the 

 beak-point, it is such a feature as might be ascribed ordi- 

 narily to a pathological growth due to the stimulus of some 

 foreign matter. Grzybowski's septum is figured as V-shaped 



