MAZATLAN UNIVALVES 365 



finely striated across ; tlie 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. S. The tuberous 

 nucleus is not seen in the adult, of which a normal 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 elongata ; Icevis, seu varie scidpta ; anfrac- 

 tibus primis abnormalibus, tuberosis, vertice submamillato, vix 

 declivi ; aperturd ad basin effusd seu angulatd ; columelld 

 haudplicatd. 

 Pars = Alaba, H. fy A. Ad. Gen. vol. i. p. 241 : (diagn. auct.) 



The species here grouped together, from their gene ral form 

 and sculpture have relations with Odostomia, Chemnitzia and 

 Eulimella ; 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 Sty lifer, 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 



