MAZATLAN UNIVALVES 473 
against that of Gray and Duclos, though not on the grounds 
on which the species was first instituted. The general style of 
painting and habit of growth varies between the Atlantic and 
Pacific shells ; but individuals will be found of each so closely 
passing into each other that the usual characters are not sufli- 
cient to distinguish them. The elevation of spire and expansion 
of the mouth are extremely variable, as will be seen by the 
measurements given below. Moreover the white colour of the 
plaits which is said to distinguish A. testacea, is sometimes 
changed to a purple brown. But when A. testacea is tinted, 
the colour begins at the base and proceeds upwards ; while in 
A. hiatula it begins above, and always leaves the last plait 
white. Again the spiral band, which is light in A. hiatula, 
is very dark in A. testacea. But the only reliable distinction 
i$ in the shape of the band and plaits. The band in A. hiatula 
begins somewhat higher up the mouth, thus occupying a larger 
proportional space over the back: and the plaits, which in A. 
hiatula are close, numerous, small, and ascending the columella 
without angle, are in A. testacea fewer, and making a slight 
angle with a few intercalary folds as they enter the shell. In 
these respects, the fossil species agrees with the African, which 
appears also from the E. coast, teste Capt. Owen, B. M. A 
shell obtained from a shop along with a number of EH. Indian 
species, agrees in almost all respects with A. testacea, but 
has the plaits more numerous, with the last fold more distinctly 
marked off, and entering the mouth at a greater angle, with 
very numerous intercalations. The surface of A. testacea is 
very minutely covered with striz of growth, and crossed some- 
times by minute close spiral lines. The same structure is seen 
on a finer scale on A. hiatula. Both species want the ordinary 
glossy aspect of Olivide. There is a conspicuous wave, a 
little below the middle of the outer lip, in the region of which 
the shell has generally a dead aspect. No opercula were found. 
The posterior canal is very deeply channeled at the junction, 
and the part of the parietal callosity which is above the suture 
has, almost always, a deep stain. The young shell is often 
prettily marked with dark olive-purple spots, which occasion- 
ally run into irregular zigzag lines. The smallest sp. is ‘65 long: 
the largest measures Jong. 1°96, long. spir. °47, lat. '86, div. 60°. 
A broad sp. heaalicoe Ve: 2954 2 ee ener!) 
An acuminate sp. ,, 1°49, : “DDS. 4, “POOs nesta 
Hab.—Acapuleo, Humboldt & Bonpland.— Real Llejos; in 
sandy mud, 6 fm.; Cwming—Panama; 20 dead sp.; C. B. 
