LAMELLIBEANCHIATA. 515 



Aotlnomya modioUformls.] 



The systematic position of Actinomya seems to be nearly intermediate between 

 Orthodesma and Modiolopsis, diflFering from the former in the somewhat shorter form 

 and tightly closing instead of gaping valves, from the latter in the thinner hinge 

 plate and^shell, and from both in the convex basal outline and absence of a mesial 

 sulcus. 



Actinomya modioliformis Meek and Worthen. , 



PLATE XXXVI, FIGS. 19 and 20. 



Modiolopsis modioliformis Meek and Wobthen, 1868. Geol. Sur. 111., vol. iii, p. 294. 

 Compare Modiolopsis superba Hall, 1861. Eep't., Sup"t. Geol. Sur. Wis,, p. 31. 



Shell of medium size, elongate, obliquely ovate, much the widest in the posterior 

 half; strongly convex. Hinge nearly straight, rather short, extending anterior to 

 the beaks almost half as far as posterior to them, and posteriorly less than half the 

 distance from the beaks to the posterior extremity of the shell. From the hinge 

 the outline passes almost imperceptibly into the oblique posterior margin, and this 

 slopes backward with a gentle convexity to the abruptly rounded posterior basal 

 extremity. Basal margin extending obliquely upward and forward, very slightly 

 convex throughout its length. Anterior end narrow, rounding sharply into the 

 extremity of the hinge. Beaks rather prominent, incurved, situated about one-sixth 

 of the entire length of the shell from the anterior extremity; a strongly rounded or 

 subangular umbonal ridge extends from the beaks to the posterior extremity of the 

 shell, the convexity becoming gradually less as it recedes from the beaks. Surface 

 with fine concentric striae and rather strong (especially on the flattened regions 

 anterior to the umbonal ridge) wrinkles of growth. Muscular impressions so faint 

 that they cannot be traced with certainty on the casts of the interior at hand. 



I believed this species to be identical with Hall's previously described Modio- 

 lopsis superba, but Prof. R. B. Whitfield, to whom a specimen was.sent for compari- 

 son with the original types of Hall's species, writes me that it is "less angular on 

 the umbonal ridge, more rounded on the base, and fuller on the lower disc." These 

 differences are probably of specific importance. Figure 20 is taken from the type 

 used by Meek and Worthen. The specimen, though a good one, is slightly distorted 

 by vertical pressure, and imperfect in front and along the base. To facilitate com- 

 parison with fig. 19, the missing parts have been restored in the figure. 



This fine species I regard as in every sens'e an Actinomya. It is, perhaps, nearer 

 A. safordi Ulrich, than any other known, but there is little likelihood of confusion 

 between them, that species being a higher shell, with a larger anterior end and 

 somewhat smaller umbones. It has also several radiating folds on the posterior 

 cardinal slope not seen in this species, 



