384 THE PALEONTOLOGY OF MINNNSOTA. 



LStrophomenldie 



Family STROPHOMENIDJ], King. 

 Genus STROPHOMENA, Rafinesque (de Blainville). 



1820. Strophomena, RAFINESQUE. Annales Gen. Sci. phys. Bruxolles, torn, v, p. 232. 



1825. StropJiomena, de BLAINVILLE. Manuel de Malacologie et Conchyliologie, vol. i, p. 513, pi. LIII, 



flgs. 2-2a. 

 1892. Strophomena, HALL. Paleontology of New York, vol. viii, pt. i, p. 245. 



Description: "Shells transversely subsemicircular or semielliptical; greatest 

 width along the hinge-line. Surface concavo-convex and covered with fine radi- 

 ating striae, which are equal or alternate in size. The pedicle valve is slightly con- 

 vex about the umbo, but becomes rapidly concave toward the middle, with the apex 

 perforated, except in old age. The cardinal area is conspicuous and nearly vertical 

 and the delthyrium closed by a convex plate or deltidium [which is internally 

 much thickened medially]. The teeth are widely divergent and are supported by 

 plates which are produced into elevated ridges nearly surrounding the muscular 

 area. The latter is relatively short, subcircular in outline, deeply excavated and 

 divided medially by a more or less distinctly defined longitudinal ridge which is 

 often continued over the pallial region. [Upon each side of this ridge in the poster- 

 ior half are situated the small, narrow adductor scars, and these are surrounded by 

 the large diductors. It is probable that the adjusters are also present outside the 

 latter, but so poorly defined as not to be recognizable.] 



"The brachial valve is concave at the umbo, becoming strongly convex with 

 growth; it has a much narrower cardinal area and the chilidium is rudimentary or 

 incomplete. Dental sockets deep and continued as narrow grooves or indentations 

 across the cardinal area. The crural plates are extended laterally with a slight 

 curve, but are not supported by septa; at their inner margins they unite to form a 

 callosity, upon which rests the short, bilobed cardinal process, which scarcely extends 

 beyond the triage-line. The muscular surface of this process is cordate in outline and 

 is placed at a low angle to the plane of the area. A low median ridge extends for- 

 ward from the hinge-plate separating two large adductor scars, in front of which 

 are two narrow elongate impressions. Vascular and ovarian markings frequently 

 well defined. Shell substance fibrous, strongly punctate. 



"Type: Strophomena rugosa Rafinesque (de Blainville), l825=Leptoena planum- 

 bon'a Hall, 1847." (Hall, op. cit.) 



The well known species of Strophomena can be separated readily into two natural 

 groups; (1) those in which the shell is concavo-convex, and (2) where the valves 

 are biconvex. The interior features are very similar in the two sections and are 

 not available for grouping. The external form of the valves, however, easily 



