LYMiNi£ID,E OF NORTH AMERICA. 295 



than humilis, with shouldered whorls and a larger aperture in com- 

 parison with the rest of the shell; the aperture is also larger and 

 rounder in galbana. Obrussa decampi has a superficial resemblance, 

 but is a much narrower shell with elongated aperture and squarely 

 shouldered whorls. The inner lip is also differently appressed than 

 either humilis, modicella or decampi. Galbana may be known by its 

 solid shell, .its swollen and somezuhat shouldered last zvhorl, its short, 

 generally dome-shaped spire and its broadly reflected inner lip which 

 cmargins the umbilical chink. The spire varies somewhat in height, as 

 does also the aperture. (See plate XXXil.) In the Michigan speci- 

 mens the sutures are not as much compressed as in the New Jersey 

 specimens. 



Like its congener, Galba obrussa decampi, galbana lived in the icy 

 waters of Post-Glacial Time, to which environment its peculiar form 

 is undoubtedly due. As remarked by Dr. Dall, the recent shells are 

 less variable, on the average, than are the fossil forms. Galbana will 

 probably be found to be one of our commonest species when it becomes 

 fully recognized ; in many cabinets it now reposes, without doubt, under 

 the names of desidiosa and humilis. I cannot agree with Dr. Dall 

 (op. cit, p. 74) in making traski Tryon a synonym of galbana. It 

 seems to me to be related to proxima. Its size of 16 mill, is far greater 

 than any galbana I have seen, and its shape, as figured and described 

 by Tryon and as shown by his type, is totally different. 



Galba petoskeyensis (Walker). Plate XXVI, figures 14-16. 



Lymncea petoskeyensis Walker, Nautilus, XXII, p. 6, pi. i, figs. 3, 5-7, May, 

 1908. 



Shell : Elongated, compressed, thin ; periostracum light yellow- 

 ish horn ; the shell is almost white and translucent ; surface shining, 

 growth lines prominent, crossed by impressed spiral lines, giving the 

 surface the wavy appearance of the typical Lymnseas ; the last whorl 

 is sometimes malleated; whorls 6-6^4, rather flat,sided, shouldered at 

 the periphery on the spire whorls, the last whorl somewhat roundly 

 compressed ; nucleus very small, rounded ; hyaline ; the first whorl is 

 very minute, the second is very large (pi. XLIX, fig. G) ; spire acutely 

 conical,, generally a trifle longer than the aperture; sutures well im- 

 pressed; aperture an elongated oval, rounded and somewhat effuse 

 anteriorly ; the posterior portion of the aperture forms an acute angle ; 

 outer lip thin, with a white internal thickening or varix within the 

 edge ; inner lip broadly reflected, flattened and emarginating the con- 

 spicuously open umbilicus; parietal callus rather thick; at the junction 

 of the inner lip with the parietal wall, the former is abruptly pressed 



