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. mcxicamis 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) (Kiister'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 A. astecorum 

 (Ph.) ! He wrote, "The epidermis, the nacre, the figure agrees 

 pretty well with U. azteconmi 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 aztecorum, approximates 

 Philippi's description of U. mexicanus. The form certainly 

 has nothing in common with P. opacata except a rather straight 

 dorsal line. 



Measure^ments 



X -ti 



X Q 



