THE NAOTILUS. 



It is to be noted that Say, witli liis usual nice observation, de- 

 scribes the minute, revolving lines wliich are still cliaracteristic of 

 the Delaware River torui, and which are not uncommon in specimens 

 from other localities. In many instances, however, they are entirely 

 lacking. Say described his species as sinistral. Without going into 

 the question as to whether the shell is really sinistral or ultra-dextral, 

 it may be said that with the exception of Say's original figures, and 

 those of Call (Rep. Dept. Geol. Ind., 1899, p. 411, pi. viii, fig. 10), 

 all the published illustrations known to the writer treat it as dextral, 

 and in accordance with this practically universal custom the figures 

 for the present paper have been drawn in that position. Those who 

 prefer to consider the shell as sinistral have only to reverse the 

 plate. 



Considering the enormous extent of territory which it inhabits, 

 and the diverse conditions of local environment to which it is sub- 

 jected in different parts of its range, P. hicarinatus is, for a fresh- 

 water pulmonate, remarkably uniform in its development, and com- 

 paratively few varietal forms have been described, and most of these 

 have been distinguished within the last few years. Taking them in 

 the order of their appearance, they are as follows: 

 I. Var. UNICARINATUS Hald., pi. I, figs. 6, 7 and 8. 

 Planorbis bicarinatus v. unicarinatiis Haldeman, Monograph, p. 7 

 (1844). 



" Whorls of the right (upper) side rounded ; the carina on the left 

 side revolves closely, so as to form a very narrow umbilicus, and the 

 aperture is much extended towards the left." (Haldeman.) 



Haldeman did not figure this form, but fortunately his type has 

 been preserved in the collection of the Philadelphia Academy, and 

 the figures here given have been drawn from it. It is very doubtful 

 whether this form is entitled to varietal rank in the proper accepta- 

 tion of the term. It will be observed from the figure that the inner 

 whorls on the upper side are distinctly angulated, and that the angle 

 does not wholly disappear until just before the aperture is reached. 

 Such specimens are not uncommon in almost any large series, and all 

 gradations from the bicarinate to the unicarinate form can be found 

 in the same colony. It would seem, therefore, to be an individual 

 rather than a racial characteristic. The form, however, is of inter- 

 est as exhibiting the first step toward the purely ecarinate form 

 described by Filsbry as var. aroostookemis. The type came from 

 the Schuylkill River. 



