THE NAUTILUS. 135 



Peristome slightly expanded, with a narrow, rusty edge ; upper and 

 outer margins very slightly expanded ; basal margin more expanded ; 

 columellar margin rather broadly dilated. The ends converge and 

 are joined by a short glossy callus. 



Alt 7.5, diam. 12.9, aperture alt. 5.8, width 6.7 mm. 



Alt. 6.8, diam. 11.6 mm. 



Alt. 6.7, diam. 11.1 mm. 



The whole upper surface, head and tentacles, are blackish-slate 

 color, finely irregularly granulose. There are no distinct dorsal or 

 genital furrows. The sole is tripartite, the areas separated by indis- 

 tinct longitudinal impressed lines, in drowned alcoholic examples. 

 The middle area is twice as wide as the others, slaty-white ; side 

 areas darker slate color. The mantle is whitish, the venation of the 

 lung outlined delicately with gray. 



The genital system (pi. XI, fig. 9) resembles that of M, hutsoni. 

 The penis (p.~) is swollen near the base, and has a slender retractor 

 muscle {p. r.), and a moderately long flagellum. The vagina is 

 very short, the spermatheca globular, its duct very long, and in- 

 serted unusually low, much farther down than in M. hutsoni. The 

 dart sack (d. s.) is large, and near its base, on the side facing the 

 vagina, the two mucus glands (m. gl.} are inserted close together (as 

 shown in fig. 10, a diagrammatic view of these organs). The mucus 

 glands descend and their enlarged ends lie near the base of the dart 

 sack. The measurements are : length of penis (to insertion of re- 

 tractor), 3 mm. ; length of epiphallus, 1.8 mm. ; length of flagellum, 

 4 mm. ; length of vagina, 1.8 mm. The jaw has about 6 unequal 

 ribs, grouped in its median part. 



This species is doubtless related to both Sonorella baileyi and S. 

 fisheri Bartsch, both of which differ in various details of sculpture. 

 It stands nearest to Micrarionta hutsoni Clapp, having the same type 

 of embryonic sculpture ; but that species has a dark band above the 

 perifery, bordered with white above, and a much larger aperture. 



Cotypes are in the collections of Ferriss and the Academy of 

 Natural Sciences (No. 94783). 



The anatomical data obtained from living examples of the species 

 hutsoni and desertorum by the junior author, indicate that we went 

 too far in referring species from the lower Colorado basin to the 

 genus Sonorella. It now seems likely that the species wolcottiana, 

 tndioensis, baileyi, fisheri, lohrii, and perhaps some others, belong to 



