22 THE NAUTILUS. 



The original specimens had been collected at several places in 

 Aroostook Co., Me., by Mr. Olaf 0. Nylander. 



Pis. costalum n. sp. (fossil). 



Mussel small, somewhat oblique, strongly inflated, with three or 

 four concentric, prominent ridges on each valve ; beaks rather pos- 

 terior, large, much projecting over the hinge margin, flattened on 

 top with a sharp, prominent concentric ridge around the flattened 

 part ; outline of the valves rather oval or ovoid, with the supero- 

 anterior slope somewhat less curved, the anterior end subangular and 

 the posterior end subtruncate ; surface with fine, irregular striae and 

 lines of growth between the ridges ; shell rather thin ; hinge rather 

 short, stout and compact, plate moderately broad, and short, cardinal 

 teeth well formed, the right slightly curved, thicker at the posterior 

 end, the left anterior large, almost straight, ascending obliquely and 

 the lamella strongly curved up, its posterior part projecting over the 

 inferior edge of the plate ; the posterior rather parallel with the an- 

 terior and extending over about two-thirds of the latter ; lateral teeth 

 close to the cardinals and the ligament, short, those of the right valve 

 stout, pointed, the outer ones very slight, especially so the anterior, 

 the grooves short and deep, the left laterals moderately stout, high, 

 pointed ; ligament short and strong. 



Size : Long 2.5, alt. 2.1 (with the beaks), diam. 2.3 mill. 



Fossil in a marl bed at Monitor, Bay Co., Michigan, in company 

 with other Pisidia, collected and sent for examination by Mr. Bryant 

 Walker. 



This species s^eems to stand near Pis. ventricosum Pr., but its beaks 

 are less posterior, and the outlines are rather different. It also re- 

 sembles P. scholtzii Cless. as described and figured, with the flattened 

 beaks. This feature, however, does not seem to be constant. In 

 two specimens of P. scholtzii which I owe to the kindness of Mr. 

 Clessin, the beaks are slightly " calyculate," but not flattened on top. 

 Also in a few younger valves of P. costatum, the beaks are less 

 flattened, and the ribs slighter. 



A PROPOSED STUDY OF GONIOBASIS. 



LAWRENCEBURG, IND., MAT, 1903. 

 EDITORS OF THE NAUTILUS : 



For many years I have been under the impression that the infor- 



