92 MAZATLAN BIVALVES 



no fewer than eleven books without a synonym ! The young 

 shell however has not been so well treated, being, according 

 to Jay and Reeve, the C. laticotitatum of Sotv. It certainly 

 suits exactly the description and measurements given in Proc. 

 Zool. Soc. 1833, p. 85. Mr. Cuming however states that C. 

 laticostatum is distinct and is the larger of the two ! The 

 young C. procerum moreover is not to be distinguished from 

 the figure of the young C. Panamense in Zool. Seech. Voy. 

 pi. 42, f. 7, which is described by Sow. Proc. Zool. Soc. 1833, 

 p. 85, as only 1*5 in. long. The specimens in the Cumingian 

 Collection however are extremely large. The Mazatlan shells 

 are manifestly of one species. It begins life nearly orbicular. 

 A young shell, *5 in. long, displays 7 close posterior ribs, 

 sharply serrated at the margin, and with rows of tubercles on 

 their summits. The 10 ventral ribs are then sub triangular, 

 rather sharp, minutely dotted at the top, and with the sides 

 and very narrow interstitial spaces finely decussated by con- 

 centric ridges which travel up and down over them. The 

 six anterior ribs are very much flattened, and faintly decus- 

 sated by the concentric lines. Gradually both tubercles and 

 concentric ridges disappear, at an earlier or later period ; the 

 ribs then become flattened, and at last on the anterior part 

 nearly pass away. The shell assumes a longer or broader 

 form, with every gradation between. The number of ribs varies 

 from 21 25, and does not depend on the shape. The colour 

 is whitish, variously spotted with reddish brown ; inside near 

 the posterior margin, (which is very ringent,) of a rich brown- 

 ish purple. The ends of the ribs project beyond the margin of 

 the shell at the posterior ventral part, and beautifully inter- 

 lock. The side teeth are in a very slight curve from the car- 

 dinal. Epidermis thin, with very closely concentric ridges. 



A broad specimen measures long. 2*5, lot. 2*2, alt. 2*04. 

 The largest,, 3'07, 2'5, 2'22. 



Hob. Real Llejos, in coarse sand 4-6 fm. Cuming. Payta, 

 TfOrbigny. Panama, rare, C. B. Adams. S. W. Mexico, 

 P. P. C. Mazatlan, Melchers. Mazatlan : common ; 

 I/pool fy Havre Coll. 



Tablet 448 contains a minute imperfect valve. 449, broad 

 variety, 5 pairs and a valve, young. 450, do. 3 sp. adult. 

 451, 4 sp. young, elongated form. 452, 3 do. adult. 453, a 

 curiously deformed valve. 



