THE CHICAGO ACADEMY OF SCIENCES. 



deferens in a depression on the side of the large sac below (which is 

 believed to be the penis) would seem to indicate that it could not be 

 vo regarded. The vas deferens forms a loop exterior to the male or- 

 gan, one end entering the penis in a depression on the side, the other 

 end entering at the junction of the penis and the appendage, reap- 

 pearing at the apex of the latter, whence its course is the same as in 

 Lymnaa, directly to the penial opening. There is no division of 

 muscles retracting the male organ in Planorbis as in Lymncea, the 

 penis in Planorbis being retracted by a wide, thin band of muscular 

 tissue which is inserted in the columella muscle near the left buccal 

 retractor, the characteristic penis retractor of Lymn&a (pi. X, A, RM) 

 being absent. It would be of great interest and value to know pre- 

 cisely the office of this peculiar appendage, but whether it prove to be 

 the true penis (and the organ below the penis sac) or an appendage of 

 the penis, does not alter the fact that its position on the penis and 

 the peculiar insertions of the vas deferens provide a characteristic 

 difference from Lymncea which is of family value. The principal 

 characters which differentiate Planorbidse from Lymnseidse are the 

 peculiarities of the male organs, the bicuspid central tooth (plate XIII, 

 B) the planorboid, discordal shell and the filiform tentacles. 



The taxonomic value of the groups Camptocerinae and Pomphola- 

 ginse of Dall, cannot be predicated at the present time, fresh material 

 not being available. Ball's figure of the . genitalia of Pompholyx 1 

 is not in sufficient detail to allow of an opinion. The central tooth is 

 bicuspid, which indicates relationship with Planorbis. 



As thus restricted, the Lymnseidse present a homogeneous group 

 of snails which may be divided into several genera and subgenera. 

 Before entering upon a discussion of the data upon which these groups 

 are based it may be of interest to review briefly some of the previous 

 classifications of the Lymnseids which bear directly upon the Ameri- 

 can fauna. All of these have been based on shell characters. 



In 1840, S. S. Haldeman monographed the genus Lymnsea, includ- 

 ing as subgenera Limnea, Radix, Leptolimnea, Bulimnea and Acella, 

 the last two being described as new. 



In 1865 W. G. Binney monographed the genus, recognizing the 

 following groups as subgenera : 



Limncea Lam., Radix Montf., Bulimnea Hald., Limnophysa Fitz., Leptolim- 

 nea Swains., Acella Hald. 



'Ann. Lye. Nat. Hist., IX, pi. 2. 



