THE LOWER SILURIAN ROCKS. 641 



middle of the valves ; pallial line simple, extending up the anterior side 

 to the umbonal cavity. 



Type: Megambonia jamesi, Meek. (Plate 48, Fig. 7.) 

 The protrusion of the byssal opening, short, edentulous hinge, and 

 non-terminal beaks are the characters that distinguish this genus from Bys- 

 soyiychia. The same features, excepting the one that relates to the 

 absence of hinge teeth, also separate the genus from Anomaiodonta and 

 Eridonyckia. The presence of a large byssal opening and the short 

 hinge sufficiently distinguish the new genus from Ambonychia as restricted 

 by me. As to Megambonia, Hall, under which genus, because of an ex- 

 ternal resemblance, Meek and others have placed the typical species 

 jamesi, it is enough to say that Allonychia is totally different internally. 

 Indeed, the two genera cannot possibly belong to the same family. 



On plate 48, figure 7 illustrates a large and well preserved cast of 

 the interior of Allonychia jamesi. Comparing it with the exterior we 

 learn that the test was thick on the anterior side both beneath and 

 above the byssal protrusion; and that the umbo is more pointed and 

 smaller in the cast than in the shell, and not so much incurved. A 

 small lobe is separated from the upper part of the byssal protrusion and 

 thus lies immediately beneath the anterior extremity of the hinge. It is 

 believed to be equivalent to a similar protuberance met with in internal 

 casts of certain species of Byssonychia (e. g. B. intermedia, M. and W. 

 sp.) and in Amphiccelia, Hall. Perhaps it is also to be likened to the sub- 

 rostral lobe of Ambonychia. Though highly improbable it is still possi- 

 ble that the cavity of which it is the filling may have lodged an anterior 

 adductor muscle. The feature should perhaps have been included in the 

 generic diagnosis. 



Allonychia ovata, n. sp. 



Plate 48, Figs. 4-6. 



The shell in this species is not so oblique (it is almost erect) and 

 relatively higher than A. jamesi. The beaks are smaller and situated 

 farther behind the anterior extremity of the shell, giving the false im- 

 pression, in a side view, of being placed quite in the middle of the hinge. 

 The anterior and posterior slopes of the surface, therefore, are more 

 nearly equal. The number of the radial ribs also is not as great, being 

 about forty and certainly not exceeding forty-five.* Though other dif- 

 ferences have been made out, those mentioned will, in connection with 

 the illustrations, suffice for the recognition of the species. 



Formation and locality : Middle beds of the Cincinnati group, about 

 325 feet above the bed of the Ohio river at Covington, Kentucky. 



* In a series of eight specimens of A. jamesi the number of the ribs varies 

 between fifty-five and sixty-eight, and in only one of these is it less than sixty. 



41 G. O. 



