ii4 STUDIES IN GEOLOGY, No. 3 



produced, straight. Umbones and general shape exactly as 

 in A. larkinii. Ribs 27 to 28 in number, prominent, rounded 

 to squarish in cross-section, tuberculate, with interspaces 

 about equal in width. The tuberculate ornamentation of the 

 ribs is best developed on the anterior surface, and loses 

 strength posteriorly, the posterior ribs not being noded. The 

 strength of the lines of growth near the margin gives the 

 valves a concentrically wrinkled appearance. The area is 

 marked by three to six wave-shaped furrows which are often 

 crossed by longitudinal lines near the hinge. The teeth are 

 fine and short mesiajly, becoming longer distally. Inner 

 margin fluted. Length, 35.5 mm. ; alt., 32.5 mm. ; diameter, 

 32.25 mm. 



This species is one of four forms from the Zorritos region 

 which are so closely related that some emphasis of distinction 

 is necessary for unambiguous treatment. Of these four, A. 

 larkinii Nelson, A. imporcata n. sp., A. obesiformis Grzyb., 

 and A. crescens n. sp., larkinii differs from all in the area, 

 and the other three form a series progressing in the order 

 named from erect, squarish forms (imporcata) through 

 more produced and more oblique forms (obesiformis) to 

 extremely produced and very oblique forms (crescens). 

 Other characteristics of these three species are more or less 

 constant, and the close relationship of the extremes to the 

 median form of the series begets the temptation to include 

 all in one species, but examination of a large number of 

 specimens shows the progressive difference in shape to be 

 sufficiently constant in each of the three species as here 

 proposed to make advisable, in the interest of clearness if 

 for no other reason, their consideration as separate groups 



A. larkinii is exactly similar to A. imporcata in every feat- 

 ure save the area. The area of larkinii is smooth, excepting 

 a series of closely set, microscopic longitudinal lines, whereas 

 that of imporcata is characteristically furrowed. Grzy- 

 bowski's statement "Die weite Area tragt ausser dachform- 

 igen Furchen deutliche dichte Querleistchen, besonders stark 



