514 THE PALEONTOLOGY OF MINNESOTA. 



[Actinomya. 



Beaks comparatively large, full and rather prominent. Umbonal ridge generally 

 strongly rounded, sometimes subangulai'. Surface with concentric lines of growth 

 and 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 scars 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. 



Type: Modiolopsis cindnnatiensis Hall and Whitfield. 



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; 6, the hinge of another right valve of the same 

 species; c 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 dndnnatiensis H. and W., M. cancellata Walcott, 

 M. pulchella Ulrich, and two undescribed species from the lower or Utica horizon of 

 the Cincinnati group, JF. subcarinata, n. sp., from the Galena, and Modiolopsis superba 

 Hall, M. 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. 8. A. Miller has described three forms having surface markings like A*tl niiitr i>1u>ltuUformts. These wn/ be 

 distinct from Hall's species, but I cannot now admit that they are. The one called M.mlcnta Is almost ci-rtainly fomnlc-ii 

 upon vertically compressed specimens of the phoiodi/ormte, while the If. eorrugata Is. so far us I can make It out, In no 

 wise different from the same species. 



