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 generally 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 radiating lines of granules. Sometimes intercalary ribs are found, much larger than the rest; sometimes different plans of sculpture are seen on the same shell. The colour outside is generally olive or brownish green ; sometimes without markings, generally 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 mottled, (? P. aeruginosa, Middendorff) 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 unfrequently with very slight markings of white, or none at all; in which case the colour is either dark greenish brown, (P. striata, Reeve. 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 slightest doubt, having carefully examined about 11,000 speci-mens, from which those here enumerated were selected by a carefully repeated process of reduction and comparison. No very young shells were found; the youngest measuring 0,5 inch in length; the largest, long.1,56 inch, lat. 1,33, alt. 0,39. Hab.— Mazatlan ; in extreme profusion;