34 PALAEONTOLOGY OF THE UPPER MISSOURI. 



Family CRASSATELLID^. 



Shell generally thick aud strong, eqiiivalve, oblong, oval, subcircular 

 or subtrigonal. Surface covered with a brownish epidermis, and often 

 ornamented with radiating or concentric cost*. Hinge with generally 

 strong cardinal teeth; ligament external or internal. Muscular im- 

 pressions usually deep and well defined ; pallial impression simple or 

 very slightly sinuous. 



Animal (in the typical genus) witli mantle lobes only united by the 

 branchial septum ; margins of inhalcnt orifices cirrhated ; foot moderate, 

 compressed, more or less triangular, and grooved. Palpi triangular ; gills 

 unequal, outer semi-lunar, inner widest anteriorly. 



Authors differ in regard to the limits of this family, some including in it several 

 genera which others refer to the YenilmW (= Cyprinida), or to some of the allied 

 families. The existing genera most generally placed here, are Astarte, Gouldia, 

 Crassatella, Cardita, Carditamera, Thecalia, Trajjezium, CoralliopJiaga, &c. It also 

 includes the following extinct groups, viz.: Venericardia, Pachydomus, Astariella, 

 Cardinia, Carbonocola, Astartila, Pachycardia, Pachyrisma, Megalodon, Mecynodon, 

 Hippopodium, Myoconcha, Opis, Pleurophorus, Cleidophorus, Erycinella, Woodia, 

 Lutetia, and probably Cyprieardella, MatJieria, Cypricardina, Anodontopsis, Cur- 

 tonotus,- &c. 



Genus PLEUROPHORUS, King. 



Synon. — Nuculites (sp.), Conrad, Ann. Report Geol. N. Y. 1841, 48. 



Pleurophorus, King, Ann. Mag. Nat. Hist. XIV, 1844, 313.— De Veeneuil, Bull. Soo. Geol. Fr. 1844 (2d ser.), 

 1. — King, Monogr. Perm. Foss. Eng. 1850, 180. 



Cleidophorus, Hall, Palieont. N. Y. I, 1847, 300. 



Clidophorus, McCoy, Paleozoic Fossils, 1852, 273. 

 Etym. — wxiupw, a rib ; te'?*. to bear. 

 Type. — Area costata, Bkown. 



Shell generally small, longitudinally oblong or subovate, inequilateral ; cardinal 

 teeth two in each valve, interlocking alternately, and more or less divergent ; 

 posterior lateral teeth one to each valve, the receiving tooth in the left valve. 

 Anterior adductor muscular scar deep, and bounded posteriorly by a ridge ; pallial 

 line simple. 



This genus may be distinguished from Cardita, by its depressed, elongate form, 

 as well as by its upper cardinal tooth in the right valve being elongated posteriorly, 

 and by having true posterior lateral teeth. From Cypricardia, and Carbonocola, it 

 differs in having the receiving tooth in the left, instead of the right valve. From 

 Carditamera, with which it agrees in the arrangement of the lateral teeth, it differs 



* The name Cyprinidse having been long in use for a family of fishes, ought not to be retained for 

 this group. 



' It is desirable that the author of this genus should select another name for it, Curtonotus having 

 been used by Hann for a genus of Crustacea, in 1835. 



