282 THE CHICAGO ACADEMY OF SCIENCES. 



shells of the same size, modicclla has five whorls, while obrussa has 

 four whorls ; in form the young of obrussa, especially of the variety 

 plica, somewhat approach modicclla. The shell is, typically, much 

 larger than modicclla, 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 Xatural Sciences of Philadelphia, Xo. 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 Einney's figure 69. The types 

 of Lea's philadclphica in the Lea collection (Xo. 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 philadclphica a variety of g alb an a, 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, Iowa ; Lemont and Joliet, 

 Illinois, and Aroostook County, Maine. Some of these variations are 

 figured on plate XXXI (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.) 



