384 THE PALEONTOLOGY OF MINNNSOTA. 
[Strophomenidewe 
Family STROPHOMENIDA, King. 
Genus STROPHOMENA, Rafinesque (de Blainville). 
1820. Strophomena,-RAFINESQUE. Annales Gen. Sci. phys. Bruxelles, tom. v, p. 232. 
1825. Strophomena, de BLAINVILLE. Manuel de Malacologie et Conchyliologie, vol. i, p. 513, pl. Lim, 
1892. Strophomena, HALL. oe ane 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 strize, 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 adjustors 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 shght 
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 hinge-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), 1825—Leptena planum- 
bona 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 
