XXXI 



Dr. Lewis endeavors, by these differences, to indi- 

 dicate respectively the genera Trypanostoma and 

 Goniohasis of Mr. Lea ; but, unfortunately, the only 

 iinportant character of distinction mentioned by him, 

 is only a sexual difference.* 



And now, having cited all that has been done 

 in the classification of these animals by American 

 and foreign naturalists, we will first ascertain the 

 sequence of the genera, and then give their names 

 and limitation as we propose to adopt them. 



Swainson commenced with the species having an 

 entire aperture, then he described genera possessing 

 a truncated aperture [Ilemisinus, Melanojjsls), and, 

 finally, those with a more or less developed channel 

 at the base. 



Dr. Gray's arrangement does not differ essentially; 

 he adds, however, GJotella, an intermediate form 

 between the Trypanostomold and Goniohasic groups. 



Messrs. Adams commence with the canaliculate spe- 

 cies, but not with the highest developed type of that 

 form, lo. They give the preference to Cerijohasia, 

 Swainson, and next give Pachycheilus, which is cer- 

 tainh' more of a Goniohasic form, and then give lo. 



Dr. Brot's " Groups " represent nearly the follow- 

 ing value and sequence in genera : Lep)toxis, Try- 

 2xmostoma, Goniohasis, Lithasia, Pachycheilus, lo, 

 Melanojjsis, Gyrofoma, JPirena. 



Mr. Lea, in remarks on his description of Gonio- 

 hasis, gives the list of genera (which we have quoted), 

 but apparently in the order of their publication. 



The sequence of genera in the foregoing examples, 

 can certainly be much improved ; lo may be consid- 

 ered as the highest development of the canaliculate 

 shell, and is also the largest in size ; we find, more- 

 over, as Mr. Lea has justly remarked, the most 

 ponderous species among the Trypanostomoi {Pleu- 

 rocerce). I would then commence with lo, and 

 proceed thus : lo, Pleurocera, Angitrema, Lithasia, 



* See Stimpson "On the Stnictiiral Characters of the so-called 

 Mfluuians of North America," Am. Jour. Sci., xxxviii, July, 18G4. 



