122 STUDIES IN GEOLOGY, No. 3 



more acutely than the anterior margin, forming an angle of 

 142 with the hinge-line. Umbones high, almost uniformly 

 14 mm. above the hinge, situated at about the anterior third 

 of the entire length, rounded in outline viewed laterally, with 

 the upper surface sloping distinctly forward in some mutants, 

 and symmetrically erect in others. Umbonal slope dome-like 

 anteriorly, circular in cross-section mesially, and with a 

 distinct surficial keel running from the posterior side of the 

 umbone to the posterior margin at an angle of about 45. 

 The keel is flared in some forms, forming a distinct shoulder 

 parallel to the margin. Ribs, 28 in number, (29 on some 

 left valves) prominent, rounded, with interspaces slightly 

 more than twice as broad as the ribs (largest ribs i mm.; 

 interspaces 2.2 mm.) showing growth-lines prominently 

 near the ventral margin, and noded irregularly, the nodes 

 being knob-like. Area high and broad, with wave-like im- 

 bricated furrows increasing in distance of separation from 

 the hinge-line to the beak and more sinuous above. Teeth 

 small and short, numbering about 85, set on a thin hinge 

 plate 36 to 40 mm. long, becoming more widely separated 

 and coarser distally. Greatest vertical length of anterior 

 teeth, 2.5 mm. ; of posterior teeth, 2.75 mm. ; of smallest 

 tooth, i mm. Inner margin regularly and deeply fluted. 

 Dimensions of two specimens: Length, 50; height, 47.5; 

 diameter, 24.5 ; Length, 47 ; height, 42 ; diameter, 20 mm. 



This form resembles A. septifera, of the local fauna, very 

 much, and is closely related to it, but the beaks are less prox- 

 imate and not so far incurved, the area is larger, and the 

 teeth are finer and more numerous. The septum noted by 

 Grzybowski 124 in his original description of septifera is not 

 present in any of a large number of specimens of toroensis. 

 A. imporcata, also of the local fauna, is similar, but is 

 smaller, with proportionally lower beaks, smaller area with 

 less varied furcation, and with decidedly more rectangular 



"*Neues Jahrb., Beil. Bd. 12, p. 633, pi. 13, fig. 2, 1899. 



