1887.] NATURAL SCIENCES OF PHILADELPHIA. 35 



of an anterior ear. It also differs in wanting the large, longitu- 

 dinal teeth which characterize true Gervillia as seen, for 

 example, in G. dlfficilis d'Orbigny, and G. anceps Deshayes.^ 

 The byssal aperture is also more clearly defined than in Gervillia. 



In these differing features it agrees approximately with Perna; 

 but it differs conspicuously from Perna in its much more elon- 

 gate form, in the distinct definition of the posterior wing, the 

 smaller number of ligamental pits, and in the crenulation of the 

 hinge. This genus is more nearly related to Gervillia than to 

 any other genus of the Aviculidse, the relation between the two 

 genera being somewhat similar to that which exists between the 

 living forms of Avicula and the Carboniferous genus Monopteria 

 of Meek and Worthen. 



The species which is described in the following paragraph is 

 proposed as the type of Dallicoiicha, but the Gervillia ensiformis 

 of Conrad is an equally typical species. The G. avlculoldes of 

 Defrance (not Sowerby) and G. solenoides Defrance seem also 

 to belong to this genus, as doubtless do several other forms 

 which have been referred to Gervillia. 

 Dalliconcha invaginata (nov. 5p.). Pi. li, figs. 4 and 5. 



Shell long and slender ; the dorsum gently concave from beak 

 to posterior end, and transversely flattened by the abrupt inflec- 

 tion of the dorsal border of each valve ; wing well developed ; 

 the anterior depression rather deep ; beaks prominent ; byssal 

 aperture moderately large, oval ; hinge-areas each bearing five 

 or six ligamental pits, which are of unequal size ; the spaces 

 between the pits marked by irregular oblique crenulations. 



Genus AGUILERIA (gen. nov.). 



Shell resembling Perna in general form, in the character of the 

 test, in its muscular markings, and in the possession of a pit- 

 bearing, diverging hinge area upon each valve. The valves are 

 more or less nearly equal in convexity ; a more or less distinct 

 ftirrow passes from the dorsal border of each valve, near the 

 apex of the beak, to the anterior margin, defining a projecting, 

 more or less inflated anterior portion of the shell, which is 

 homologous with the anterior ear of Margaritophora. The 

 beaks are not prominent, situated anteriorly, but not terminal ; 

 ligamental pits distinct, but not numerous. The articulating 



^See Paleont. Francaise, Ter. Ciet., iii, pi. 39-1, fig. 3; and pi. 396, fig. 7 



