26 University of Michigan 
1914). As the type figure is that of a female specimen, a male 
specimen is here figured in some detail (plate VI). 
U. mexicanus Philippi (1847) apparently has been confused 
with this species by various authors. Crosse and Fischer began 
the trouble by placing the two in the same section, without 
comparison. Von Martens (1900) remarks that U. opacatus 
is “perhaps only a shorter variety of U. mexicanus.’ Simpson 
(1914) apparently considered U. mexicanus as practically 
unidentifiable, but had a shell like a young opacatus that he 
thought satisfied the description of the former. A careful 
examination of Philippi’s somewhat blurred, but rather good 
figure (1849) (Kuster’s copy as usual is abominable), and a 
comparison of the description and proportions, will, I believe, 
convince the most skeptical that U. mexicanus is exactly what 
Philippi (one of the keenest observers of his time) intimated 
that it was: a rather distinct form related to 4. aztecorum 
(Ph.)! He wrote, “The epidermis, the nacre, the figure agrees 
pretty well with U. agtecorum and at first I held this form 
(Art) for a variety of the same, yet there occur the following 
differences. . . .” (translation). A specimen in the Wheatley 
collection (A. N. S. P.), labeled azgtecorum, approximates 
Philippi’s description of- U. mexicanus. The form certainly 
has nothing in common with P. opacata except a rather straight 
dorsal line. 
MEASUREMENTS 
4“ 
a cB) 
= Ae SS 
eS os 38 
S =I ce 
= Kal Cy) Oy 
fo +» Oo 4 oO 
y cs eG Oe 
Dey oe 
5 Oo oS oS 
a ae ja 
P. opacata Reh Oy 52 (C.and F., 1894) 
A. mexicana 64 58 38 (Philippi, 1849) 
Means (male type) BB OM 44 
Means (female type) 50.)—Cts«#S5 47 
Extremes (58 
specimens) 43-66.5 58-75 40-54 
