80 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



represented, both by typical, or very nearly typical, forms, and by others fall- 

 ing into the different subgenera. Even the curious subgenus Calloarca 

 appears to be cpiite nearly, if not exactly, represented by such Jurassic forms as 

 Cuculhra discoris of Quenstedt. This genus also continued through the Creta- 

 ceous and Tertiary periods, and is well represented in our existing seas, where 

 the typical section, as well as the group Acar, probably attain their greatest 

 development. Poli/nema anil Plagiarca were founded on Cretaceous types, 

 while the types of Striarca and Granoarca are from the Miocene Tertiary. 



Barbatia (Poly 11 em a!) parallcla, Meek. 

 Plato 2, fig. 10. 

 Arcal parallela, Meek (1872), Haydeu's Second Ann. Keport U. S. Geol. Survey of the Territories, 303. 



Shell small, longitudinally oblong, being about twice and a half as long 

 as high, moderately convex; cardinal and pallial margins straight and nearly 

 parallel; anterior side short, rounding up regularly from below, and inter- 

 secting the cardinal margin at an obtuse angle above; posterior side long, a 

 little wider than the other, with its margin compressed and obliquely truncated 

 above, but rounded below ; beaks depressed, somewhat flattened, incurved, 

 not very remote, and placed about one-fifth, the length of the valves from the 

 anterior margin; cardinal area very narrow, and apparently smooth, or only 

 marked with one or two longitudinal cartilage-furrows; muscular and pallial 

 impressions very obscure; hinge with denticles longest posteriorly, where 

 they are directed upward and backward at an angle of about forty-five 

 degrees to the cardinal margin; from the posterior side they diminish rather 

 rapidly in size and length forward, so as to become very minute and 

 crowded between the beaks, which is as far forward as they have been traced 

 in the specimens examined. Surface showing very fine, crowded, radiating 

 strise, with stronger marks of growth. 



Length, 0.95 inch; height, 0.37 inch; convexity, 0.27 inch. 



In first describing this species, I referred it provisionally, with a mark of 

 doubt, to the genus Area, but distinctly stated that it does not belong to that 

 genus as restricted by modern conchologists. A more critical examination of 

 the typical specimen leads me to believe that it also has denticles at the an- 

 terior end of the hinge like those behind, excepting that they are directed 

 obliquely forward and upward. The hinge-margin also seems to be a little 

 widened before, as behind. From these characters, 1 think there is not much 

 room for doubt in regard to the propriety of referring it lo the genus Barbatia; 



