304 
characters undoubtedly do often, but not always,. hold good. In a 
L. arctica from Iceland (Fig. 7 a). I found the dentition to be ex- 
actly intermediate between those of the tvvo forms regarded os nor- 
a b c 
Fig. 7. Littorina obtusata. Nat. size. Iceland. 
a The variety arctica Møller, b Intermediate form between L. obtusata f. typica and 
the variety arctica. c Form with short, obtuse spire. 
mal (Fig. 8 a). It agrees with L. obtusata in the above named first 
character (Fig. 8 b), and with L. arctica in the second character 
c 
Fig. 8. First lateral tooth of Littorina obtusata. 240 /i. 
a The variety arctica. (Fig. 7 a) Hamarsfjordr, Iceland. b Forma typica, Vestmannaeyjar. 
c The variety arctica , Hamarsfjordr, Iceland. 
(Fig. 8 c). Since it is a wellknown faet that in the arctic and bo¬ 
real zones, forms are often found intermediate between L. obtusata 
f. typica and L. arctica as regards the shell, e. g. Fig. 7 b, we are 
lead to consider L. arctica as a northern variety of L. obtusata. 
The name Nerita littoralis, Linné, has caused mueh confusion. 
Forbes & Hauley (7), Jeffreys (14), Fischer (16), Tryon (24) etc. 
