92 MAZATLAN BIVALVES 
no fewer than eleven books without asynonym! The young 
shell however has not been so well treated, being, according 
to Jay and Reeve, the C. laticostatum of Sow. It certainly 
suits exactly the description and measurements given in Proce. 
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. Beech. Voy. 
pl. 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, ‘5in. 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 subtriangular, 
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, lat.2°2, alt. 2°04. 
The largest ,, i BOM ek es Ost eer 
Hab.—Real Llejos, in coarse sand 4-6 fm. Cuming.—Payta, 
D Orbigny.—Panama, rare, C. B. Adams.—S. W. Mexico, 
P. P. C.—Mazatlan, Melchers.— Mazatlan: common ; 
P’pool & 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. i 
