110 AMERICAN JOURNAL 



Canal curved backward from Canal curved more or less 



the plane of the aperture. strongly to the left of the aper- 



ture. 



Outer margin of the mouth Outer margin lighter than 

 darker than the throat. the throat. 



On the whole, I can add little to the admirable differential 

 diagnosis given by Judge Cooper in his description of 0. Mid- 

 dendorfii. He had but one specimen of the Western form, which 

 he compared with one hundred and thirty specimens of the 

 Eastern shell. From a comparison of a nearly equal number 

 of both species, I have come to the same conclusions. It should 

 also be remembered that liratus does not occur on the Arctic 

 shores of North America, and the two species are separated by 

 a vast expanse of water. 



Some of the characters in the comparative table graduate 

 toward each other in exceptional cases, but the sum of the 

 characters is always sufficient to discriminate between the two, 

 and this is all that can be expected between any two nearly 

 allied forms. I regard the two as perfectly distinct. 



MURICIDiE. 

 Genus PURPURA, Brug. 



Purpura, Brug., Enc. Meth. i, pp. xv and 241, 1789. Lam., 

 Syst. An. sans Vert. p. 77, 1801. 



Type Purpura persica, Lam., 1. c. 



Subgenus Purpurella, Dall. 



Shell purpuroid ; aperture contracted, posterior and anterior 

 sinuses obsolete ; outer lip strongly dentate, not furnished with 

 spines or digitations ; inner lip concave, flattened, with one or 

 more distinct spiral ridges or plaits upon the columella. 



Type Purpura [Purpurella) columellaris, Lam., An. s. Vert. 

 vii, p. 236, 1822. 



Habitat, Galapagos Islands. 



This "very singular" shell, as Lamarck called it, was considered 

 by Swainson to form the passage from Purpura to Ricinula, and 

 has been the subject of remarks from almost every author who 

 has referred to it. It is somewhat surprising that it has not 

 been sectionally separated before this time. It is quite as distinct 

 from the typical Purpura, with its unarmed outer lip, simple 

 columella, and effuse aperture, on the one hand, as Monoceros is 

 on the other. The contraction of the peristome is very marked 



