406 THE PALEONTOLOGY OF MINNESOTA. 
[Rati uesquina alternata 
defined; socket [crural] ridges very small and uniting behind the cardinal process to 
form a deltidium ; [adductor] muscular scars comparatively small, but deeply impressed 
near the cardinal process on each side of a small, short, mesial ridge, and nearly 
surrounded by a low obtuse ridge formed by. a thickening of the adjacent internal 
surface of the valve; anterior and lateral margins more or less thickened and genic- 
ulated within (especially in adult shells), the thickened zone being transversely fur- 
rowed [by the vascular sinuses], and sometimes granular, while outside of it the imme- 
diate edge of the valve is suddenly flattened and minutely striated and granulated. 
"Ventral valve a little convex at the umbo, but generally much compressed 
over the whole visceral region in the adult (which includes the whole surface of the 
young and half-grown shell), but becoming more convex (sometimes strongly so) 
anteriorly, or antero-centrally and laterally, and thence more or less curved up to 
the anterior and lateral margins; area of moderate hight, flat and directed obliquely 
backward nearly at right angles to that of the other valve; beak very small, scarcely 
distinct from the margin of the area, and minutely perforated; foramen broadly 
triangular and arched over above by the deltidium, which is very deeply sinuous on 
its inner edge, the sinus being nearly or quite closed by the dental process and del- 
tidium [chilidium] of the other valve. 
"Interior with cardinal margin somewhat carinate within; hinge teeth moder- 
ately prominent, remote and widely divergent; dental ridges obscure and extending 
obliquely outward and forward, but not produced or curving to surround a saucer- 
shaped cavity for the muscular scars; scars of the adductor muscles narrow, long 
and closely approximated, or almost in contact; those of the cardinal [diductor] 
muscles on each side very large, fan-shaped, but shallow, separated sometimes by a 
small ridge in advance of the adductor scars, and marked by radiating furrows and 
ridges, while the anterior and lateral regions are usually marked by striae and scat- 
tering granules. 
" Surface of both valves ornamented by numerous radiating striae, that increase 
in number, on the ventral valve, mainly by intercalation, and are usually an-anged 
with one to six or eight smaller and shorter ones between each two larger and more 
prominent ones, the largest one of which often occupies the mesial line, while on 
the dorsal valve they more frequently increase by division and are generally of more 
uniform small size. On well preserved specimens all the radiating lines are crossed 
by numerous very minute, regular, closely arranged concentric striae, that are invis- 
ible without the aid of a magnifier; a few moderately distinct subimbricating marks 
are also seen near the free margins of adult shells." 
A comparison of the interior of Trenten specimens with those from the Hudson 
River group shows that the latter have all the parts more strongly developed, owing. 
