62 MEMOIRS OF THE CARNEGIE MUSEUM. 



specific name of delectus {I. c, p. 52, PL 2, lower fig.; PI. 3, u^Dper fig.). This is a 

 synonym of cicatricosus. 



I think that Frierson makes another mistake in identifying Lea's U. varicosus 

 (Obs. I, 1834, PL 11, fig. 20) with his detedus. Lea's species is very doubtful, 

 and the figure is possibly inaccurate in several respects; but chiefly in that the 

 sculpture is entirely different from that of the species of Plethobasus (continuous, 

 concentric, rather regular ridges, not confined to the middle of the shell). I have 

 shown above that Lea's species is, if anji^hing, an exceptional individual of Fus- 

 conaia subrotunda (Lea) (See above, p. 9). 



Frierson's U. cicatricoides (p. 53, PL 2, upper fig.) is also only an extreme 

 variation of cicatricosus, distinguished by less oblique shell, and less anterior 

 beaks. It connects this species with cooperianus. I have a specimen (St. Marys, 

 West Virginia), which answers well to Frierson's figure. A specimen from the 

 same localitj^ is more distinctly oblique, and stands actually midway between the 

 two figures on Frierson's PL 2, and comes very close to Call's figure of U. varicosus 

 (1900, PL 55) except that it is not so much drawn out at the lower posterior end. 

 And further certain individuals of P. cooperianus, collected by myself, very closely 

 approach in outline the first one, just mentioned, but the sculpture is more dis- 

 tinctly and typically that of cooperianus. 



Finally these intergrading forms also show certain relationships to P. cyphyus, 

 which will be discussed below. 



Plethobasus cooperianus (Lea) (1834). 

 Quadrula cooperiana (Lea) Simpson, 1914, p. 852. 



Plate V, fig. 5. 

 Records from Pennsylvania: 



Rhoads, 1899 (Ohio River, Beaver, Beaver Co.; and Coraopolis, Allegheny Co.). 

 Ortmann, 19096, p. 198. 



Characters of the shell: Shell moderately large, heavy. Outline subrotund, 

 subovatc, or subtriangular, about as long as high, slightly oblique. Beaks moder- 

 ately high, more or less inclined forward, but not at the anterior end of the shell. 

 Beak-sculpture not observed, but probably poorly developed, and not extending 

 to any considerable degree upon the disk, since the latter is always smooth near 

 the beaks before the tubercles begin. Shell moderately swollen, rather evenly 

 convex upon the sides, without a posterior ridge. Posterior slope very slightly 

 compressed, in young specimens sometimes with an indication of a wing. Disk 

 covered with nodes or tubercles, which, however, always leave free the anterior 



