364 MAZATLAN UNIVALVES 
Genus TRUNCATELLA, Risso. 
423. P TRUNCATELLA ——-——, sp. ind. 
Tablet 1721 contains an extremely young shell and an older 
fragment, which may belong to this genus. The young shell 
looks like an Aclis; but the vertex, as in Tr. Montagui is not 
sinistral, but mammillated, with the first whirl sunken and 
searcely oblique. Tt has three and a half normal and very 
tumid whirls, scarcely diverging; and is very much smaller 
than the corresponding state of Tr. Montagui, only measuring 
long. ‘043, lat.°015. The fragment is *027 across, with a con- 
tinuous peritreme, large umbilicus and smooth exterior. 
Hab.—Mazatlan ; off Spondylus, extremely rare; L’pool Col. 
Famity PLANAXIDA. 
Genus PLANAXIS, Zam. 
424, PLANAXIS NIGRITELLA, Forbes. 
Proe. Zool. Soc. Dec. 1850, p. 273, pl. 11, f. 6. 
=Planaxis acutus, Mike. in Zeit. f. Mal. Nov. 1850, p. 169, 
no. 23. 
+ Planaxis obsoletus, Whe. loe. cit. p. 170, no. 24. 
Although Menke’s names have a month’s priority, yet as the 
description of Forbes is much more accurate, and represents 
the whole species, while Menke only indicates the extreme 
forms, it is here preserved. It is given on the authority of the 
Kellettian collections as from San Juan Del Fuaco: but as it 
is an extremely abundant shell, and yet was not found, either 
there or along the Californian coast, either by Mr. Nuttall or 
the Transatlantic collectors, it is probable that the specimens 
were collected at San Juan in the Gulf of California, or even 
at Mazatlan. That they are identical with the Mazatlan species 
is proved on comparison by the remarkable epidermis, which 
lies in an irregular mass, grooved in a comb-like manner 
diagonally ; and when this is removed, there appears a fine 
velvety coat lying in the interstices. This peculiarity, by 
which it may easily be separated from the very similar P. semi- 
sulcata of the W. Indies, is found in both of Menke’s forms, 
which pass into each other by insensible gradations. The 
nucleus has three tuberose whirls, with a flattened apex, very 
