392 THE PALEONTOLOGY OF MINNESOTA. 
(Strophomena rugosa (Rafinesque Ms). 
striated| on their posterior faces and directed very obliquely forward and outward; 
socket ridges short and oblique [but much thickened, and upon them are placed the 
linear and but slightly elevated crural plates]; mesial ridge low [in some specimens, 
usually strongly elevated and rounded], extending but a little distance forward, while 
the space between it and the socket ridge, on each side, is occupied by a moderately 
distinct muscular scar. [Vascular trunks and genital markings faintly indicated 
and similar to those in S. incurvata.| 
“Ventral valve broadly and rather deeply concave in the central and anterior 
regions, and slightly convex at the beak, which is very small, abruptly pointed, scarcely 
projecting beyond the edge of the area, and usually [always] minutely perforated; 
area moderately high, extending the whole length of the hinge, generally but little 
sloping laterally, flattened and inclined more or less backward, foramen [delthyrium]| 
closed by a prominent, rounded deltidium, that is transversely striated and rather 
broadly sinuous on its inner edge, for the reception of the [chilidium, which partially 
covers the posterior portion of the] cardinal process of the other valve. Interior 
showing hinge-teeth to be well developed, trigonal and striated on their posterior 
sides, while from their inner bases the dental laminz extend forward so as nearly to 
encircle the usual saucer-shaped depression for the [diductor and probably very small 
adjustor| muscular scars, which is sometimes [always more or less] divided by a small 
linear mesial ridge [upon each side of which are situated the narrow adductor scars]; 
cardinal margin prominent and sharp within, on each side of the hinge teeth; anterior 
and lateral regions more or less thickened within and roughened by the crossing of 
the vascular markings, which are scarcely visible on any part within this zone. 
“Surface of both valves ornamented by numerous fine, closely-crowded, radi- 
ating striz# that are often alternately a little larger and smaller or, on some parts, 
with several of the smaller ones between each two of the larger; the smaller being 
always shorter than the larger, or ending at various distances between the free mar- 
gins and the beaks without coalescing with those between which they are intercalated. 
Striz and furrows minutely crenulated by extremely small, very regular, closely- 
arranged, concentric lines, invisible without the aid of a magnifier; a few subimbri- 
cating marks of growth are likewise seen near the free margins.” (Meek, op. c7t.) 
The comparative length and width of the shell vary considerably in this species, 
the latter being in some cases eqyal to two-thirds, in other cases only about one- 
half of the former. The narrow specimens with the long hinge-line have received 
the name S. elongata James, but a large collection will show every gradation between 
this and S. planumbona or rugosa. Some specimens are nearly as long as wide and 
are often difficult to separate from S. nutans (James’ Cat.) Meek. The latter, however, 
in its typical condition, is very distinct and approaches S. fluctuosa Billings. 
