MAZATLAN UNIVALVES 



473 



againsi that of Gray and Duclos, tliougli not on tlie grounds 

 on which the species was first instituted. The general style of 

 paiati'ig 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 suffi- 

 cient to distinguish them. The elevation of spire and expansion 

 of the mouth are extremelj' 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 ui^wards ; 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 

 is in the shape of the band and plaits. The band in A. hiatula 

 begias 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 E. Indian 

 species, agrees in almost all respects with A. testacea, but 

 has the plaits more numerous, M-ith the last fold more distinctly 

 marked ofi", and entering the mouth at a greater angle, with 

 very numerous intercalations. The surface of A. testacea is 

 very minutely covered with strise of growth, and crossed some- 

 times by minute close spu-al lines. The same structure is seen 

 on a finer scale on A. hiatula. Both species want the ordinary 

 glossy aspect of Olividfe. 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 long.l'Q^, long. spir. "47, lat. '86, div. 60". 



A broad sp. „ 1-6. „ -29, „ 76, „ 70«. 



An acuminate sp. ,, 1'49, ,, "55, ,, "58, ,, 40". 



Sah. — Acapulco, Humboldt Sf JSonjiland. — Keal Llejos; in 

 sandy mud, 6 fm. ; Cuming. — Panama; 20 dead sp. ; C. B. 



