THE ATLANTIC SLOPE ARCAS 



By Pkarl G. Sheldon 



This treatment of the Areas* is intended to include: the synonymy, description, 

 and distribution of the Tertiary and recent species of the genus which occur in the 

 beds along the coast of the eastern United States; also references to the Cretaceous 

 species, to the deep water forms which occur off the coast, and to the recent and Ter- 

 tiary species of the Caribbean district. 



In measuring the specimens they were placed on crossection paper with the hinge 

 along a horizontal line and the point of the beak on a vertical line. Measurements 

 were taken from the Hne through the beak to the line touching the anterior point of the 

 shell; from the line through the beak to the line touching the posterior point; from the 

 hinge line to the highest and to the lowest points of the shell. Distances anterior to the 

 beak and those above the hinge line were marked +, and those posterior to the beak and 

 below the hinge line — . The diameter was measured at the widest part of the shell. 

 Semidiam. means the diameter of a single valve. 



Because of the radiating sculpture and convexity of the shell, in photographing the 

 specimens it was usually impossible to bring out the details of sculpture with equal dis- 

 tinctness over the entire surface. In practice, the detail usually photographed best at 

 the end away from the source of light. Ordinarily in this genus the sculpture is stronger 

 toward the anterior and weaker toward the posterior end, therefore the specimens were 

 usually photographed with the posterior end nearer the light in order to give a compara- 

 tively true figure. Likewise, the angle of the teeth on the hinge varies and some cast 

 little or no shadow. For the same reason the crenulations along the ventral margin 

 were often lost. The muscle scars photographed indistinctly. In general, detail was lost 

 where the light rays were perpendicular to the surface of the shell or tended to be par- 

 allel to radial sculpture, crenulations, etc. The Hghting was arranged to bring out the 

 most iniportant specific characters. Unless otherwise stated, the figures are natural 

 size. 



Dall (Wagner Free Inst. Sci., Trans., vol. 3, pt. 4, 1898) placed all the species under 

 the genus .4 rca and ■g'a.vQ. Barbata, No'ctia, Scapharca, etc., which at various times have 

 been given the rank of genera, the rank of subgenera and groups or sections. These di- 



*The major part of this paper formed a thesis presented to the faculty of the Graduate School of 

 Cornell University in partial fulfihiient of the requirements for the degree of Doctor of Philo.sophy. 

 Sincere thanks are due Professor G. D. Harris for aid, sugpestions and criticism. Nearly all of the 

 Tertiary material used was collected hy I'rofes.sor Harris or his students. 



