MAZATLAN UNIVALVES 365 
finely striated across ; the whirls then rapidly enlarge, and are 
at first smooth, then more or less spirally grooved. The mouth 
in the young shell is elongated, as in young Cerithia; and in 
the very early stage, the general appearance is that of Litiopa, 
some species of which also have the first whirls abnormal. ‘The 
shell varies in the greater or less elevation of spire, strength 
and frequency of spiral ridges, and in colour from brown to 
nearly black. The operculum is very thin ; slightly spiral, as 
in Nerita ; with the apex rather further off from the extremity 
than is shewn in the figure in the P. Z.§. The tuberous 
nucleus is not seen in the adult, of ached anormal specimen 
measures long. ‘48, long. spir. °22, lat. ‘33, div. 55°, The 
youngest specimen is (05 long. 
Hab.—‘‘Straits of Juan del Fuaco,” Forbes. [? |—Mazatlan ; in 
extreme profusion ; L’pool Col. 
Tablet 1722 contains a series of 262 specimens, obtained by 
repeated elimination from the examination of many thousands, 
and representing every observed variation of age and form ; 
also several loose opercula. 
Genus ALABA, H. & A. Ad. 
Testa ovoidea seu elongaty ; levis, seu varie sculpta ; an ifrac- 
tibus primis abnormalibus, tuberosis, vertice submamillato, vie 
declivi; aperturd ad basin effusd seu angulatad ; columella 
haud plicata. 
Pars=Alaba, H. & A. Ad. Gen. vol. i. p. 241: (diagn. auct.) 
The species here grouped together, from their general form 
and seulpture have relations with Odostomia, Chemnitzia and 
Kulimella; but are separated from them by the want of sinis- 
tral apex, which seems to be compounded-for by the abnormal 
character of the first three or four whirls. These are always 
different from the rest, either in sculpture, divergence or both; 
and generally present the appearance of a tuberous root. In 
this respect they have relations with Stylifer, from which they 
differ in the straightness of the axis, the want of sinistral apex, 
and the strong sculptured habit of many of the species. In 
some respects they resemble Litiopa; but differ in habit of 
growth, and in the want of Achatinoid truncation of the base. 
Not knowing any genus in which they could consistently 
remain, I had described them under the provisional name of 
Tuberia. But as the typical species is clearly congeneric with 
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