204, MAZATLAN UNIVALVES 
This species, and its more temperate analogue, A. patina, go 
through the same varieties of form and pattern ; some of which 
are so divergent that it is not to be wondered at that they 
have been described as distinct ; and others are so marvelously 
alike that they can scarcely be separated from each other by 
figure or description. The habit however of the two species is 
sufficiently distinct; and those who have examined large 
multitudes of specimens will have little difficulty in separating 
them. The simplest guide is the prevailing green and brown 
colour of this species, and the prevailing white and black 
of the other. The character of the ribs, which is mainly relied 
upon by Middendorff, appears subject to great variation. 
Shell extremely variable in colour and markings, but gene- 
rally rather broad and flat, with the apex somewhat inclined 
anteriorly, especially in the young shell. Outside with the 
apex and sometimes a considerable portion of the shell nearly 
smooth; generally with extremely fine ribs, sometimes sharp, 
sometimes rounded, generally slightly granulose ; sometimes 
with broad strong ribs ; sometimes nearly smooth with radiat- 
ing lines of granules. Sometimes intercalary ribs are found, 
much larger than the rest ; sometimes different plans of sculp- 
ture are seen on the same shell. The colour outside is generally 
olive or brownish green ; sometimes without markings, gener- 
ally with white lines either radiating or broken up; often with 
white patches tessellating with the brown; or changing from 
one pattern to another. Inside the shell is generally whitish 
about the middle,- (whence the name,) with more or less of a 
bluish green tinge, sometimes dark green, sometimes brownish, 
sometimes with an element of ochre yellow more or less mot- 
tled, (? P. eruginosa, Mid.) There is almost always a large dark 
spot at the body mark, of a brownish olive green, in which 
sometimes the brown, sometimes the dark green predominates. 
The body stain is irregularly and slightly gathered into points : 
the head mark is generally shewn by a stain shaped like a 
sector, bounded by two radii from the apex, about 70° apart. 
The margin is generally broad, occasionally very narrow, 
bounded inside by a greenish line; ordinarily tessellated with 
brown and white, sometimes with green or yellow; not un- 
frequently with very slight markings of white, or none at all: 
in which case the colour is either dark greenish brown, (P. 
striata, Rve. sp. 69, non Quoy, sp. 58), or with intermediate 
stages to very light greenish white. That all the shells here 
classed together belong to the same species, I have not the 
