28 THE NAUTILUS. 



cular aspect, and in the young are much obscured by the polish of 

 the base of the shell ; base rounded moderately, about as much as 

 the spire, slightly depressed, with a very thin, brilliantly polished 

 callus near the axis ; aperture with the upper lip projecting consid- 

 erably beyond the lower one, moderately thickened and rounded, 

 overrunning the keel at the inner corner where there is a narrow, 

 sharp sulcus, of which the termination in fully adult shells makes a 

 a decided notch in the edge of the lip ; lower lip receding, flexuous 

 slightly thickened ; throat with three basal, one axial and two par- 

 ietal, strong, subequal, spiral laminee, much as in C. salleana, the 

 pillar very short, rendered flexuous by the end of the keel ; the in- 

 ternal walls of the preceding whorls and most of the axis, absorbed. 

 Lat. of base (major) 30, (minor) 26, alt. 11 mm. 



Habitat, Pilitla, San Luis Potosi, Mexico, E. W. Nelson. 



This is the finest species of the genus, more evenly divided by the 

 keel, more depressed, and larger than C. salleana or C. eolina, the 

 only species hitherto known. 



SOME OBSERVATIONS ON THE GENITAL OEGANS OF UNIONIDJE, 

 WITH REFERENCE TO CLASSIFICATION. 



BY DR. V. STERKI. 



(Concluded^) 



" Margaritana." Considerable changes will be necessary about 

 those species hitherto ranged under this genus, and some evidently 

 related forms, e. g. Unio pressus Lea and Anod. edentula Say. The 

 latter two species seem to stand near Marg. truncata, rugosa and 

 complanata. In all of them, the soft parts are of rather the same 

 appearance, and especially so are the branchiae, of which the outer 

 are gravid, in almost their whole extent, from fall to spring. U. 

 presses, Marg. rugosa and A. edentula were found with the posterior 

 halves of the branchiae empty evidently just emptied, the anterior 

 part still filled with young, in spring. Some of them were seen with 

 the branchiae empty, the gonads charged, in July. 



In edentula, the young are arranged in a singular way, apparently 

 different from others. There are small, cylindrical, worm-like, whit- 

 ish masses, of about one mill, diameter, lying transversely in the 

 brauchise, closely packed together. In them, the young are located, 

 six to ten or more in each one, in single or double file, each one in 



