MAZATLAN UNIVALVES 201 
young of P. Mexicana it has much closer analogies. The 
largest specimens of P. pediculus however do not at all run 
into the smallest of P. Mexicana. They have all the appear- 
ance of being old shells, with the margin narrow and the 
shape long and irregular; while P. Mexicana, as it is traced 
upwards, displays a very wide semitranslucent margin, and a 
broad regular shape, with the ribs not rounded and prominent 
but simply giving an angular form to the shell. To settle the 
point satisfactorily will require a series of P. Mexicana from 
its earliest stage, as well as a knowledge of the animals. Of 
this species I have carefully examined nearly 300 specimens. 
The youngest shell that has its ribs developed and clearly 
belongs to this species is ‘17 long. Tablet 912 contains two 
smaller ones without distinct ribs, which probably belong to it. 
Even when very young, they are almost always incrusted with 
corallinous matter. The largest specimen measures long. 1°3, 
lat. 1°, alt.°4. The ordinary size however is very much smalier 
and flatter. 
Hab.—Mazatlan, Philippi.—Acapulco, Reeve.—Mazatlan; not 
uncommon ; L’pool & Havre Coll. 
Tablet 913 contains 3 young specimens.—914, 3 sp. ordinary 
state.—915, 3 do. ribs well developed.—916, 3 do. light tortoise- 
shell within.—917, 3 do. dark tortoiseshell.—918, 3 do. broad 
variety.—919, 4 do. ribs numerous.—920, 4 do. elongated, ribs 
small.—921, 1 sp. abnormally elongated, with very small ribs.— 
922, Patella discors, with place of attachment of P. pediculus. 
261. PatTEeLzta piscors, Phil. 
Abbild. und Besch. Conch. pl. 2, f.6—Rve. Conch. Ic. sp. 78, 
pl. 29, f. 78, a, 5. 
The peculiar characteristic of this species, viz. the crumpling 
near the umbo, is very seldom seen without careful cleaning, 
as the shell is almost always thickly coated with vegetable 
corallinous deposits, alge, &c. The surface is extremely finely 
ribbed throughout, but at a little distance from the black apex 
the shell is gathered into irregular undulations, about 15 (more 
or less) which soon disappear, and leave the shell henceforth of 
a regular growth. It goes through all the changes common to 
limpets, bemg sometimes high, sometimes depressed; some- 
times suborbicular, generally oval. The margin is extremely 
finely crenated, and is generally more or less dotted with black. 
The prevailing colour is a greenish white, with more or less 
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