200 MAZATLAN UNIVALVES 
Shell extremely large ; white, or of a yellowish or greenish 
tinge, occasionally orange; with a broad marginal band of a 
semidiaphanous hue, very conspicuous in the young shell. 
Muscular sear in adult raised, irregularly lobed and corrugated. 
The outside is frequently covered with Alge and bored by 
Tithophagi. Its surface is a favourite place of adherence for 
smaller limpets. In the young shell may be traced about 10 
very indistinct principal ribs, with a profusion of radiating 
lirule. These however very soon disappear. The youngest 
ascertained specimen measures Jong. 1°65, lat. 1°4, alt. °42. 
The‘largest sp. a eee te ae eee 
A flatter do. Teas! ee Pay ba By os iis tH 8 
Hah. — Mazatlan, ee Voyage.— Do., Menke. — Do. ; 
abundant, L’pool § Havre Coll—Monterey, Col. Jewett, 
(Gould ms.: non Nutt.)—Payta, Peru, D’ Orbigny. 
Tablet 908 contains 3 young sp. different ages.—909, 1 sp. 
finely grown, adolescent, margin flattened.—910, 1 do. margin 
sharp, muscular scar thick, brownish red.—911, the largest sp., 
outside riddled by Lithophagi. 
260. Patetna PEDIcoLUs, Phil. 
Zeit. f. Mal. 1846, p. 21, no. 8. 
=P. corrugata, Rve. Conch. Ic. sp. 132, pl. 40, f. 132, a, 6. (1855.) 
Comp. P. Araucana, B. M. Cat. D’ Orb. Moll. p. 53, no. 448: 
(=however P. zebrina, var., teste Gray in loco.) 
Shell normally flat, oblong, solid, with 10 stout rounded ribs 
projecting at the margins, of which 2 are in the axis of length 
with 4 on each side: ribs and interstices radiately striated : 
yellowish white, generally with more or less of black or brown 
tortoise-shell markings within, sometimes with the black be- 
tween the ribs as described by Phil. and Rve. Sometimes the 
shell is more rounded and the ribs rather angular, in which 
state it might be taken for the young of P. Mexicana. Occa- 
sionally a Siow other intercalary ribs appear. Inavery few 
unusually large specimens, the ribs are nearly obsolete at the 
margin and the shell is much lengthened. The body mark 
varies as usual; when plain, it is gathered into points as in P. 
discors. The very young shells appear not to develop the ribs 
marginally, in which state they might be taken for the young 
of P. discors. The stout ribs of the adult shell however bear 
no analogy with the very finely marked surface of the latter 
with its curiously puckered circum-umbonal portion. With the 
