104 PALAEONTOLOGY OF THE UPPER MISSOURI. 



If we disregard the distinctions based upon the presence or absence of a circum- 

 scribed cardinal area (and there appear to be various intermediate gradations in 

 this character), the foregoing six groups may probably be reduced to four, by 

 uniting the "Flabellatce" with the " Multicostatae," and the " Cardissaides" with the 

 " Bucardince." As thus arranged, these groups would apparently correspond nearly 

 with what are usually regarded as subgenera, in conchology. 



There is, however, another little group, differing, it is believed, from all of those 

 denned by Prof. Agassiz, though probably nearest the " Ovales." It is only known 

 in the Cretaceous rocks, and was called Cymella by the writer, in the Smithsonian 

 Check List of North American Cretaceous Fossils, 1864, p. 34. The type is Pho- 

 ladomya undata, Meek and Hayden, a small oval, subequilateral, rather compressed 

 shell, with very regular concentric undulations (like those of Inoceramus), crossed 

 by impressed radiating lines, only visible near the middle of the valves ; cardinal 

 area distinct. 



Mr. Conrad has also proposed two subgenera under Pholadomya, viz., Anatimya 

 and Margaritaria. The type of the first is his Pholadomya anteradiata, a Creta- 

 ceous species ; and the other Avas found upon his P. abrupta, from the Miocene. 

 We are not acquainted with these shells, but from the -figures and descriptions, 

 incline to the opinion that they should be regarded as the types of distinct genera, 

 particularly the latter. 



The genus Pholadomya was probably first introduced during the Liassic period, 

 and attained its greatest development during the deposition of the later members 

 of the Jurassic system. It was also well represented in the Cretaceous, and some 

 two or three species have been described from the Tertiary rocks. A single species 

 only (the type of the genus) is known to inhabit our existing seas. It is found on 

 the shores of the island of Tortola, West Indies. 



Some apparently closely allied forms are found in the Triassic, Permian, and 

 Carboniferous rocks, but they all want the radiating costa? of the true Pholadomyas, 

 and are referred to Allorisma, Myacites, Platymya, Chcenomya, &c. 



Pholadomya hiimilis. 



(PLATE IV, Fig. 3, a, 6.) 



Pholadomya humilis, MEEK & HAYDEN, Proceed. Acad. Nat. Soi. Phila. May, 1860, 182; ib. Oct. 1860, 418. 

 Shell longitudinally oblong, ventrieose. Posterior end rounded, and more or leas gaping ; base nearly straight 

 along the middle ; anterior end very short, narrowly rounded below the beaks. Dorsum nearly parallel with the 

 base, slightly concave in outline ; escutcheon lanceolate, and bounded by an obscure angle on each side. Beaks 

 depressed, gibbous, incurved, and located in advance of the middle. Surface ornamented by small, regular, con- 

 centric wrinkles, crossed by a few raised lines, or small, distant costae, which radiate from the back part of the 

 beaks to the posterior and postero-basal margins. 



Length, about l.OG inch ; height, 0.47 inch ; breadth, 0.52 inch. 



As far as can be determined from our imperfect specimens, this species seems to 

 be new. It bears some resemblance in form to P. subelongata, Meek (Trans. Albany 

 Institute, vol. IV, p. 42), from rocks of Cretaceous age on Vancouver's Island, but 

 its beaks are more depressed, and its radiating costre more distant, as well as more 

 obscure. 



Locality and position.. Lower part of the Jurassic beds at southwest base of 

 Black Hills. (Type No. 217.) 



