70 THE NAUTILUS. 



anterior in the young, well prominent over the hinge margin, slightly 

 inclined towards the anterior, moderately large, rounded in the adult, 

 but with a small, more or less flattened area near the center, in some 

 specimens bounded by a slight ridge, most noticeable in the young ; 

 surface with moderately coarse, subregular sulcation, rather sharp to 

 shallow, rugulose, slightly shining, with one or sometimes two well- 

 marked, dark lines of growth ; color generally grayish in the adult, 

 lighter over the beaks and with light, yellowish zones along the mar- 

 gins, often with dark mottlings ; in some specimens there are irreg- 

 ular zones of bluish ; shell moderately thick, substance white, muscle 

 insertions distinct, those of the protractor pedis separated from the 

 anterior adductors ; hinge moderately stout, curved, formed rather 

 as in Sph. solidulum Pr., plate narrow, cardinal teeth short and 

 slight; ligament rather long, covered, or a narrow median line un- 

 covered in old specimens. 



Soft parts, in alcohol, pale, the syphons with a yellowish tinge ; 

 palpi large ; outer branchiag comparatively small ; metapodium dis- 

 tinct, but rather small, with the retractor pedis ; branchial cavities 

 with numbers of young at different stages, the largest 5 mill, long, 

 seems to be free in the cavity (not in a marsupiuni). 



Size : long. 17.5, alt. 14, diam. 9.5 mill. (= 100 : 80 : 54.3); one 

 specimen 18 mill. long. 



Habitat : Water holes, Crow Creek, 25 miles N. E. of Greeley, 

 Colorado, collected by Mr. Junius Henderson, curator of the museum 

 of the University of Colorado. There were over 120 specimens in 

 the lot, of all stages of growth, few of them full-grown ; some also 

 were in alcohol. 



So far as can be judged, the present Sphcerium is distinct from all 

 described species, and except for the largest forms of S. simile Say, 

 is the largest. The mussel is higher than in simile, the beaks nar- 

 rower and more prominent, the shell and hinge are stouter, the sul- 

 cation coarser, and the color and surface appearance quite different. 



There is a Sphcerium from several parts of the Mississippi valley, 

 somewhat smaller and more inflated, and partly of different outlines, 

 regarded as distinct since 1896, but shelved. It may be a form of 

 the present species, which was named in honor of its discoverer. 



A few specimens were affected in the same way as those of S. sol- 

 idulum Pr. from Iowa, recently described in the Nautilus. 



