LAMELLIBRANCHIATA. By) 
Actinomya modioliformis.] 
The systematic position of Actinomya seems to be nearly intermediate between 
Orthodesma and Modiolopsis, differing 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. 
ActrnomyA mopioLtirorMis Meek and Worthen. 
PLATE XXXVI, FIGS. 19 and 20. 
Modiolopsis modioliformis MEEK and WORTHEN, 1868. Geol. Sur. Ill., vol. iii, p. 294. 
Compare Modiolopsis superba HALL, 1861. Rep’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 ffasses 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 striz and rather strong (especially on the flattened regions 
anterigr 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 sense an Actinomya. It is, perhaps, nearer 
A. saffordi 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, 
