THE LOWER SILURIAN ROCKS. 



657 



traces of the posterior sear, and radial lines on the ventral slope. The 

 latter, as is the case also with other species of the genus, never show on 

 the exterior of the shell but are strictly an internal feature and rarely 

 noticeable except on well grown or old examples. 



Associated with this species, at any rate with the shell that every 

 collector at Cincinnati has identified with Hall and Whitfield's M. cincin- 

 natiensis, another is found, though much more rarely, which may have 

 been included by them in their M. cincinnatiensis and perhaps used in 

 the illustration of their species. The posterior end of this shell has an 

 outline really very much like their figure 14, being subangular at the end 

 of the hinge and but little curved in the oblique slope from that point to 

 the sharply rounded post-basal angle. While I can believe readily that 

 specimens of this second form may have been included among those to 

 which they applied the name cincinnatiensis, I cannot understand, con- 

 sidering the attention they were obliged to give it in making a drawing 

 of it, how the} 7 could have failed to notice the radiating lines which trav- 

 erse the cardinal and posterior parts of the surface and are distinguisha- 

 ble even on all the interval casts seen by me. Such radii are not often 

 to be seen on the common form, but it is important to know that they do 

 exist occasionally, though always fainter than in the rare one. 



6 



Figs a and b. Actinomya cancellatal Walcott, sp. 



In the above cut a represents the cast of a small left valve, and 

 b a natural mould of the exterior of two imperfect valves joined together 

 by the external ligament, which, together with the hinge, is retained by 

 the specimen. Comparing these figures with figure 16, plate 56, certain 

 differences appear aside from the posterior radii. Thus the anterior end 

 is relatively a little longer, narrower and less regularly rounded, the post- 

 cardinal angle is better defined, the posterior margin less curved except be- 

 low where it turns more abruptly into the basal line, and the umbonal 

 ridge more prominent and better defined. These differences, though 

 slight, are constant, and, coupled with the greater distinctness of the 

 posterior radii, it seems reasonable to consider them as of specific import- 

 ance. 



Granting that the two forms are distinguishable, whether as species 

 or varieties is immaterial, the question arises, to which of the two is the 

 42 G. O. 



