H2 STUDIES IN GEOLOGY, No. 3 



"Shell thick and heavy. Anterior extremity short and 

 rounded, posterior more or less produced. Beaks widely 

 separated, raised and very prominent. Ligament area large, 

 about half as broad as long. Surface marked by from 30 to 

 33 radiating ribs, which are rounded and broader than the 

 spaces between them. Ribs ornamented by rounded tubercles 

 and crossed by numerous fine lines of growth. Teeth num- 

 erous, strong, nearly straight, equidistant, except at the 

 extremities of the hinge line, where they become divergent 

 and much stronger. The margin of the shell is deeply 

 scalloped by the extremities of the exterior ribs and grooves. 

 * * * Anterior muscular scar almost circular; posterior 

 elongated and narrow." Nelson, 1870. 



Examination of Nelson's specimens shows the anterior 

 margin to be almost straight, and curved only where it meets 

 the ventral margin, which is straight and almost parallel 

 to the hinge-line. The posterior margin is little if at all pro- 

 duced ; the shell is subequilateral and has a squarish aspect 

 viewed laterally. The anterior surface is rounded, with a 

 hint of a keel ; the median surface is flattened, and the pos- 

 terior slope is flat to slightly excavated, being separated 

 from the median surface by a prominent keel which drops at 

 an angle approaching the vertical from the umbone to the 

 margin, where it meets the apex of the posterior marginal 

 angle. The area is smooth except for very fine longitudinal 

 lines, and is set off from the rest of the shell by a deep 

 marginal furrow followed by a thin smooth band, which 

 is sometimes indented by a subsidiary furrow. The teeth are 

 in two series, vertically disposed mesially, and oblique dis- 

 tally, the anterior distal teeth being shaped. Length, 27.5. 



Exact locality and horizon not known. Although no speci- 

 mens of A. larkinii occur in the Hopkins collection, it is so 

 striking a form, and offers so many interesting features 

 inherently as well as through its relationships to other Areas 



