60 



lines, about twelve in number, which cross the concentric ribs, but which 

 do not give a nodular aj^pearanee to the sculpture. These radiating stria^ 

 can only be seen in a rather strong light ; they are most conspicuous on 

 the ujiper surface of the valves, and become obsolete near the base, also 

 at the anterior end. 



As the above description was made from a mere cast, it is probable 

 that the sculpture of the test was much more decided than it is in the 

 only sjiecimen now accessible. 



Greatest length, about nineteen lines ; maximum height nearly twelve 

 lines ; extreme width or thickness, eleven lines. 



This fossil belongs to the second of Agassiz's divisons of the genus, the 

 " Pholadomj^es avec un Aire Cardinale Circonscrite,"* also to the fifth 

 section of that group, the " Pholadomyes Ovalaires."f 



Among European forms its analogies are with such species as 

 P. modlolarisX ^^^^ P- ovulwn^ of Agassiz, particularly with the 

 latter. In the first, a broad groove traverses the valves obliquely near 

 the anterior end, and this is not present in P. ovuloides ; in the second, 

 the hinge line is not produced much beyond the umbones in front, nor is 

 the anterior margin angular above ; the oj^posite being the case in 

 sj^ecimens of P. ovuloides. 



In the Cretaceous I'ocks of Noi-th America there are several species 

 of Pholadomya, which bear a considerable resemblance to P. ovuloides, 

 and the Pholadomya papyracea of Meek and Hayden, || from the Upper 

 Cretaceous of Chippeway Point near Fort Benton, on the Upper 

 Missouri, in particular has a very similar shape, and almost exactl}^ 

 the same sculpture. Still, P. 2>'tpyracea is a much more compressed 

 shell than P. ovuloides; the anterior end of the former is not angular 

 above ; and the hinge margin of P. papyracea is " not inflected so as 

 to form a defined false area." The inflection of the hinge margin of 

 P. ovuloides is veiy decided, and this feature alone will serve to dis- 

 criminate between the two siiecies. 



* " Etudes Critiques sur les Mollusques Fossiles." Livraison II., page 103. 



t Idem, page 113. + Idem, page 123. Plate III. a, flgs. 1—6. 



§ Idem, page 119, Plate III., figs. 7—9. Plate III. /-, figs. 1 -0. 



II " Proceedings of the Academy of Natural Sciences, Philadelphia, 1862," page 28. Also, "Report 

 on the Invertebrate Cretaceous and Tertiary Fossils of the Upjier Missouri Country." By F. B. Meek, 

 Washington : 1870. Page 217, Plate V., flgs 4, a, b. 



