354 Monogra-pli of the Species of 



sometimes an additional small one on the transverse lip, to the 

 left of the constant one ; and the teeth on the outer lip vary 

 in many ways, as may be best exhibited by the following ta- 

 ble of varieties which I have observed, intended to represent 

 the teeth in succession as they occur, beginning at the outer 

 posterior angle and going round to the left. 



By this table it appears that there may be as many as even 

 eleven teeth, though I have never seen more than eight on 

 one individual. Of the two teeth which are shown by this 

 table to be constant on the outer lip, the largest is situated 

 nearly opposite the tooth on the transverse lip, and the other 

 less than half the distance between it and the posterior angle. 

 Such remarkable deviations have led to the proposal of two 

 new species which may with propriety be noticed, and may be 

 considered as the two extremes of the varieties, viz. : 



P. Tappanidna, Adams, a name adopted by him from the 

 manuscripts of the late Dr. Ward of Roscoe, Ohio, and de- 

 scribed in a supplementary sheet to the 40th volume of Silli- 

 man's Journal, and afterwards in Thompson's History of Ver- 

 mont. The finest examples of the species come under this 

 variety. They are such as occur in Ohio, of a very light 

 color, quite translucent, without epidermis, and with the lip 

 distinct and white. It is ^^^ inch in length, and about half as 

 broad, fig. 10. 



P. carvidens, Gould, (Inverteh. of Massachusetts, 189, fig. 

 120) is about one seventh smaller, has a very thin, greenish 

 epidermis, and generally eight teeth, and the large one on the 

 transverse lip decidedly curved to the left, (fig. 11.) Such are 

 specimens usually found in Massachusetts ; and though in 

 size, exterior, and armature they are so diflfercnt from the 



