514 THE PALEONTOLOGY OF MINNESOTA. 
[Actinomya. 
Beaks comparatively large, full and rather prominent. Umbonal ridge generally 
strongly rounded, sometimes subangular. Surface with concentric lines of growth 
aod often with radii or divaricating folds; the radii sometimes restricted to the 
inner side of the shell, showing on casts of the interior and not on the exterior of 
the shell itself. Muscular sears and pallial line as in Modiolopsis, excepting that in 
the majority of the species they are very faintly impressed. Hinge plate edentulous, 
very narrow, especially so under the beaks, a little wider and grooved on each side 
for the reception of a linear internal ligament. A similar external ligament prob- 
ably also present. y 
Type: Modiolopsis cincinnatiensis Hall and Whitfield. 
Penn 
X 
c 
G ‘a. 2) 
G=Ssan 6 
Fig. 39. a, a large right valve of Actinomya cincinnatiensis, mostly devoid of shell, showing the 
muscular scars and delicate internal radii on the cast; b, the hinge of another right valve of the same 
species; ¢ and d, hinges of a left and a right valve of Actinomya pholadiformis Hall, sp. The student 
will’do well to compare these hinges with those of Modiolopsis and related genera, figured on a suc- 
ceeding page. 
This genus brings into very natural association a number of Lower Silurian 
species, the described forms of which have heretofore been placed chiefly with 
Modiolopsis, These are Modiolopsis cincinnatiensis H. and W., M. cancellata Walcott, 
M. pulchella Ulrich, and two undescribed species from the lower or Utica horizon of 
the Cincinnati group, A. subcarinata, n. sp., from the Galena, and Modiolopsis superba 
Hall, MW. modioliformis Meek and Worthen, and Orthodesma saffordi Ulrich, from 
the lower limestone of the Trenton formation. 
Besides these, I propose to place here another group of species, so far known 
only from rocks above the Trenton, that approaches Modiolopsis in the strength and ; 
definition of the anterior adductor impression, while differing from that genus, and 
therein giving us a clue to their origin, in the convexity of the basal outline and 
absence of a mesial depression or so-called “byssal sulcus,” and in the character of 
the hinge, which is thinner, and thus more like that of an Orthodesma than of species 
of Modiolopsis of the same size. Four species of this kind, all from the Cincinnati 
rocks, are known to me, only two of them, however, being described, i. e., Modiolopsis 
pholadiformis Hall, and M. oblonga Ulrich.* 
*Mr, S. A. Miller has described three forms having surface markings like Actinomya pholadiformis. These may be 
distinet from Hall’s species, but I cannot now admit that they are. The one called M. sulcata is ulmost certainly founded 
upon vertically compressed specimens of the pholadiformis, while the M. corrugata is, so far as I can make it out, in no 
wise different from the same species. 
