THE NAUTILUS. 53 



of the species (i. e., is not an average specimen). The best figure of 

 this shell, though itself poor, is that given by Kiister (Taf. 58, fig. 2). 

 In its general facies the shell somewhat resembles the Unto pyra- 

 midatus, with the addition of the characteristic elevations. Mr. 

 Swainson mentioned that he had seen a specimen of mytiloides with 

 pustules down the center. There can be no doubt that he had a 

 shell of this species before him. The specimen figured was given 

 me by Dr. Ball, and came from the collection of the National 

 Museum. It measures, length 87, alt. 70, diam. 39^ mm. 



UNIO CICATRICOIDES, new name. PI. II, upper figure. 



This is the shell figured and described by Conrad in his Mono- 

 graphy (page 115, plate 64), and also by R. E. Call, Indiana Mol- 

 lusca (Indiana Geological Report, 1900), plate 55, to which descrip- 

 tion and plate reference is herewith made. Its general facies is 

 much more like aesopus than detectus, from which latter it differs in 

 being more inflated (especially behind) and is a larger shell ; its 

 lateral teeth are much more club-shaped and larger; the posterior 

 sinus of the hinge margin is nearer the end of the ligament; the 

 color of the epidermis is also different from that of delectus. Length 

 93, alt. 69, diam. 55^ mm. 



UNIO COMPERTUS, new species. PI. Ill, middle and lower figs. 



Shell medium in size; length 2.3, height 2.1, diameter 1.3 inches. 

 Apparently dimorphic, the females (?) being broader behind than the 

 males and more rounded, the males (?) being somewhat triangular 

 and pointed behind, beaks high and well forward (their sculpture not 

 seen). Epidermis dirty yellow, darker before (as in circulus*). 

 Basal outline rounded, and in the females expanded in the middle; 

 shell not very much inflated. The posterior ridge is rounded, and 

 becomes more and more inflated with age. The posterior area is 

 narrow, with several more or less well-defined lines from beak to 

 margin. Down the center of the disc runs a row of pustules, larger 

 in the females, as well as more numerous. Inside, the nacre is 

 white, quite thick in front, as far back as the center, or row of pus- 

 tules, from thence it becomes remarkably thin in comparison, pro- 

 ducing a trough-like excavation from beak to posterior base. Teeth 

 erect and fairly stout; two cardinals and two laterals in the left valve 

 and one each in the right. 



