114 LAND AND FRESH WATER MOLLUSKS 



tooth is large, longer than wide, truncated above, expanded below its 

 middle, and incurved at the basal margin. The reflected portion is 

 large, tricuspid, the cusps prominent. The laterals have a long, nar- 

 row base of attachment, a small portion of its upper part thrown out- 

 ward, the rest curving inward, giving an irregular arcuate form to the 

 base as a whole ; the anterior and posterior margins of this base are 

 abruptly truncate. The reflected part is rather posterior and carries a 

 large, wide, expanding, bluntly truncated cusp on the outer side, and 

 on the inner a very small conical cusp. The successive teeth laterally 

 from the middle of the radula at first increase, then gradually decrease 

 in size, but retain essentially the same characters to the outer termina- 

 tion of the row. 



From the typical Onchidium {schrammi Bland and Binney, 

 Guadeloupe, W. I.) the teeth differ by the wider rhachidian, with 

 more nearly equal cusps, by the presence of two distinct cusps on the 

 laterals, and by the curve of the lateral bases, which in O. schramtni 

 have their posterior portions curved toward the center of the radula, 

 while in O. borealis the curve is in the opposite direction. In Onchi- 

 della Jloridana Dall, an oculiferous agnathous species from Knight's 

 Key, Florida, the discrepancy of the rhachidian cusps and the curve 

 of the lateral bases agree with O. schrammi^ but there is a small 

 accessory inner cusp to the laterals. 



Mr. Binney informs me that the liver in O. borealis is in fasciculi 

 of long caeca, one on each side ; there is also an accessory lateral 

 pouch to the stomach, which also has a fasciculus of caeca, making 

 three biliary ducts. 



According to Semper this species agrees in most respects with the 

 fifth of the groups into which he divides Onchidium. There is a 

 single row of large glands which open through equally spaced small 

 tubercles on the mantle edge. The other glands, which in the other 

 species (except O. celtica) empty on the under surface of the mantle, 

 are absent in this form. The penis is short and thick, consisting of 

 two well marked portions. In the posterior thinner part a short 

 broad penial papilla is present, at the base of which the spermatic 

 duct opens. The wall of this part is marked by extremely shallow 

 grooves in which concretions are present, very like those found in the 

 deep grooves of other species. The spermatic cord is short and feebly 

 twisted. The penial retractor muscle is thin and attached proximally 

 to the middle of the pericardial sac on the inner surface of the foot. 

 The jaw and radula are as described by Binney. 



I should like here to record my dissent from the ingenious hypothesis 



