450 Annals of the Carnegie Museum. 



end, with a small flattened area on top, each with a slight to somewhat 

 lamellar ridge in the middle of the nepionic shell;- surface dullish, 

 microscopically rugulose, with rather fine, sharp, regular to subregular 

 concentric striae; color pale corneous; shell rather strong, subtrans- 

 lucent; hinge well curved, very stout, plate broad; cardinal teeth 

 small, the right angular with the posterior part thick and deeply 

 grooved; below it is a deep excavation for the left anterior, with the 

 edge of the plate raised over its general level; left anterior placed 

 somewhat obliciuely and well above the edge of the plate, strongly 

 curved upward, pointed; the posterior short, oblique, slightly curved; 

 laminae stout, strongly rugulose, their cusps barely or slightly pointed, 

 moderately abrupt; ligament and resilium short, stout. 



Measurements.- — Long. 2.6; alt. 2.4; diam. 1.6 mm. (100 : 92 : 69). 



Habitat. — Chains, Wabash River, Posey County, Indiana, collected 

 by Mr. A. A. Hinkley in 1908, and it is with pleasure that I name the 

 species after him. Type specimens are in the collection of Mr. Hinkley 

 and in the Carnegie Museum, No. 7,857. Although only a few speci- 

 mens are at hand, from young to full-grown, they are sufficient to 

 show that the species is well characterized and distinct, belonging to 

 the same group {Rivtilina) with F. compressum, punctatiim, etc. 

 From the former they differ by the much smaller size, the shape, and 

 having the hinge well curved, but not angular. From punctatiim they 

 differ as follows: they are larger, more oblique, the beaks being more 

 posterior and more prominent; the superior margin is more curved, 

 the hinge is much stouter. The specimens were in company with a 

 few P. punctatiim simplex (full-grown, long. 1.5; alt. 1.4; diam. 1.2 

 mm.). 



- It appears to be in place at this point to state that the ridges on the beaks of 

 all Pisidia of this group: P. compressum, supinum, fallax, cruciatum, punctatum, 

 henslowanum, etc., are not at the inferior margins of the nepionic valves, but at 

 about their middle; they are parallel with the lines of growth, and in most instances 

 highest in their posterior parts, and thus the beaks of the mature mussels appear 

 more pointed. In P. henslowanum the ridges are near the posterior margin, and 

 thus oblique, appearing almost radial. The ridges are formed by a curving-out, 

 or bulging, of the shell, concave inside in the very young mussels, and consequently 

 are not "appendages," as they once were generally termed. There was a time 

 when the ridges on the beaks were regarded as the distinguishing and characteristic 

 feature of P. compressum Prime; but on the one hand there are a number of mani- 

 festly distinct species (outside of supinum and henslowanum) having ridged beaks, 

 and on the other hand there are some forms of P. compressum without any ridges. 



