146 The Molluscan Family Planorhidae 



lateral teeth are also narrower. In pilsbryi (plate 56, fig. 3) and injra- 

 carinatum (plate 56, figs. 4, 5) the laterals are more quadrate, similar to 

 those of trii'olvis, but the marginals show some variation. In truncatum 

 (plate 57, figs. 1, 2) the laterals are narrower (as in chautauquense and 

 winslowi) . In several laterals of truncatum, there was observed to be con- 

 siderable splitting of the entocone and ectocone cusps (see fig. 2, plate 57, 

 teeth 1, 5-10). In the large magnificum, the intermediate teeth are very 

 long and narrow, the ectocone is placed high up on the margin, and the 

 extra cusps of the entocone appear later m the series (plate 57, fig. 4, 

 21-26). The marginal teeth are also narrow and the cuspidation is different 

 from that in trivolvis and other species of Pierosoma (fig. 4, 27-41). In 

 corpulentum, the teeth are arranged in a similar manner to those of 

 trivolvis (plate 57, fig. 3). The races vermilionense and multicostatum 

 (plate 58, figs. 2, 3) are similar in form. In whiteavesi (plate 58, fig. 1), 

 the teeth are similar in form to those of the corpulentum group. 



In the western group of Pierosoma, Helisoma plexatum has narrower 

 lateral teeth than trivolvis (plate 59, fig. 1). In specimens from another 

 locality, however, the early laterals are quadrate (fig. 2). In specimens of 

 plexatum from Rexburg, Idaho, many of the laterals, and some of the 

 marginals, in one specimen had the mesocone broken and wavy and 

 otherwise modified (plate 58, fig. 4). Other individuals were normal (fig. 

 5). A lateral and two intermediate teeth of a specimen from Smartweed 

 Lake, Colorado, are shown in fig. 6 on plate 58. A form of subcrenatum 

 from Pass Lake, Puget Sound, Washington, has quadrate teeth, similar to 

 those of trivolvis (plate 59, fig. 5). Another form from Lost Lake, Puget 

 Sound, had very wide, low lateral teeth (plate 60, fig. 1). In one specimen 

 from this locality, the center tooth of the eighty-eighth row had seven 

 small cusps and the fifth lateral had two sharp entoconic cusps (plate 60, 

 fig. 2). Specimens from Skagit County, Washington, were normal, with 

 quadrate laterals (plate 60, fig. 2). 



Typical subcrenatu)n from Cottonwood Pass, near Gypsum, Colorado, 

 is shown on plate 61, fig. 2. There are eight subquadrate lateral teeth and 

 four intermediate teeth. The marginal teeth are normal as to cuspidation. 

 An allied species, horni, has a large, somewhat obliquely formed mesocone 

 on the lateral teeth, which are not as quadrate as in subcrenatum (1-9), 

 the intermediate teeth are narrow and the entocone and ectocone are not 

 broken into smaller cusps (10-14). The marginal teeth (15-32) are less 

 cusjiidate than similar teeth in subcrenatum. The outer marginals (32) are 

 small and more or less vestigial (see plate 60, fig. 7). 



The large traskii from Kern Lake, California, is peculiar in several 

 respects (plate 60, figs. 4-6). A nearly complete row (the ninetieth) is 

 shown in fig. 5. In the center tooth, the two large cusps extend a consid- 

 erable distance below the lower margin of the base of attachment. The 

 lateral teeth (1-8) are longer than wide and are unmodified tricuspid. The 

 intermediate teeth (9-10) have the entocone and ectocone split into small 

 cusps. The marginal teeth (11-22) arc wide and the denticulation is very 

 irregular, small interstitial cusps appearing among the larger cusps in some 

 teeth (as 11, 13, 14, 17). In another row (eightieth), the last intermediate 

 tooth and the first marginal tooth were peculiar in this respect (fig. 6). In 

 the eighty-second row of another sjiecimen, this feature was well marked 

 (% 4). 



