282 THE CHICAGO ACADEMY OF SCIENCES. 
shells of the same size, modicella has five whorls, while obrussa has 
four whorls; in form the young of obrussa, especially of the variety 
plica, somewhat approach modicella. The shell is, typically, much 
larger than modicella, parva and the other members of the humilis 
group. Within certain limits, obrussa is very easily recognized and 
need not be confounded with any other species. 
The types of obrussa, two specimens, are preserved in the Acad- 
emy of Natural Sciences of Philadelphia, No. 58700. They are both 
broken, but one specimen is sufficiently perfect to settle the question 
of its specific identity. This specimen corresponds with Say’s de- 
scription and is fairly represented by Binney’s figure 69. The types 
of Lea’s philadelphica in the Lea collection (No. 118687) are identical 
with common forms of obrussa. They have five whorls, which are 
somewhat shouldered, the sutures are compressed and there is a small 
umbilical chink. The specimens are from Center City, Pa. Dr. Dall, 
in his Alaska Mollusks, makes philadelphica a variety of galbana, but 
Lea’s specimens belong unquestionably to obrussa and have only a 
superficial resemblance to galbana. Some small individuals ‘resemble 
galbana in general form, but have a more acute spire and a differently 
shaped inner lip and aperture. The series of figures on plate XXXI 
illustrate this similarity in form and also show the general transition 
into typical obrussa. In galbana the spire is always short and wide, 
the outer lip is much more arched at the upper part, and the inner lip 
lacks the impressed character as it meets the parietal wall, a feature 
very marked in obrussa. Lea’s acuta was at first thought to be a 
recognizable variety of obrussa, but the study of extensive series from 
different localities shows that it is a minor variation of obrussa. This 
form is typically of a pronounced fusiform shape, with elongated 
spire and aperture, the latter being notably effusive at the anterior end. 
Lea’s types comprise three specimens, of which the largest was figured 
by Binney. (Fig. 70.) There are six full whorls and the columellar 
plait is quite heavy. Several lots of obrussa have been examined in 
which the variations may be traced from a short rounded shell to the 
fusiform aspect called acuta by Lea. As these forms are all associated 
together in colonies spreading over a territory scarcely a yard in area, 
they can scarcely be regarded as varieties. This variation has been 
observed in sets of shells from Des Moines, lowa; Lemont and Joliet, 
Illinois, and Aroostook County, Maine. Some of these variations are 
figured on plate XX XI (especially fig. 20 and fig. 33, which are good 
examples of acuta). Certain large, robust specimens of obrussa have 
been identified as pallida Adams. (See under pallida.) 
