34 PROCEEDINGS OF THE ACADEMY OF [1887. 



exposed, and it appears to have been divided into an outer and 

 inner portion by a calcareous septum; umbonal furrows distinct, 

 producing an emargination at the posterior part of the convex 

 basal border and a considerable prominence of the jwsterior 

 extremity; hinge strong; the lateral teeth slender and extending 

 the full length of the lunule and escutcheon respectively; surface 

 marked by strong concentric furrows and ridges, which end 

 abruptly at the margins of the lunule and escutcheon respectively; 

 the surface of both lunule and escutcheon plain, 



AVICULIDJE. 



Genus DALLICONCHA (gen. nov.). 



Shell resembling GervUUa in general form, in the character of 

 the test, in the 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; beaks terminal, 

 divergent; the upper borders of the hinge areas converging from 

 the widely separated beaks to the posterior end of the wing, 

 where the areas come in contact with each other by their full 

 width; posterior wing elongate, clearly defined from the body of 

 the shell; anterior wing absent, the anterior extremity of each 

 valve being inflexed so as to form, when both valves are together, 

 a three-lobed depression in the front portion of the shell, one 

 lobe of which ends at the extremity of each of the divergent 

 beaks and the other below, at the contact of the antero-basal 

 margins of the valves. At the bottom of the depression there is 

 a distinct byssal aperture, to form which both valves are nearly 

 or quite equally notched. The articulating portion of the hinge 

 of each valve is marked by more or less distinct creuulations 

 which cross it obliquely downward and backward, and which are 

 sometimes visible upon the surface of the areas above the articu- 

 lating border. At the anterior end of the hinge these creuula- 

 tions are approximately perpendicular, and sometimes denticulate 

 in character, and at the posterior end they sometimes assume the 

 form of slender, nearly horizontal lateral teeth, above which are 

 more nearly transverse creuulations. 



Dalliconcha agrees with Gervillia in the characteristics already 

 mentioned; but it difters from the typical forms of that genus 

 mainly in the inflection of the anterior extremity of the valves, 

 and the consequent terminal position of the beaks, and absence 



