204 MAZATLAN UNIVALVES 



Tliis 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 Ls niainly relied 

 upon by MiddendorflF, appears subject to great variation. 



Shell extremely variable in eolovir and markings, but gene- 

 rally rather broad and flat, with the apes 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 difterent 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. aeruginosa, 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, Bvc. sp. 69, non Quo?/, sp. 58), or with intermediate 

 stages to very light greenish white. That aU the shells here 

 classed together belong to the same species, I have not tlie 



