242 MAZATLAN UNIVALVES 



continues the subsutural line ; and the other, which is most 

 conspicuous on younger shells, being often nearly evanescent 

 in larger specimens, appears on the spire, Thft labrum iu 

 this species also resembles Adeorbis ; and iu one specimen is 

 slightly thickened, producing a few varical ridges near the 

 aperture. Another has the idtimate whirl abnormally depress- 

 ed. Long. -025, lat. "Ooo by -045, div. 160". 



iZai.— Mazatlan ; 4 perfect and 1 broken specimens off Chama 

 and Spondylus ; L'pool Col. 



Tablet 1153 contains a beautifully perfect specimen, and a 

 larger one with the varical mouth. 



303. VlTKINELLA BIFKONTIA, n. S. 



V. t. .suhdiscoidea, subdiaphana, nitente, pulcherrimd, ai\fraC' 

 tibus iv. et dimidio : carinis duabus Icevibus, maxime proniin- 

 entibus, angulatis, spiralibus, qxiarum una infra, una supra 

 peripheriam sitm sunt ; carina tertid inter eas, ad peripheriam, 

 parum conspicud ; carina quartd posticd, hand proninente, 

 prope suturam valde impressam, in spird decurrente ; ad urn- 

 bilicum maximum angulatd, haud carinatd ; basi subplanatd, 

 striis duabus spiralibus, intiis quadratim deeussatis, exfus derus- 

 satione remotiore ornatd ; ad sjiiram striis incrementi irregu- 

 laribus, rvgulis intus umbilicum szibcrenantibus ; aperturd 

 guadratd, a carinis angulatd, haud nisi ad carinam basalon 

 anfr.penvM. attingente ; spird subplamitd, anfr. ult. decumbente. 



Variat carinuUs spiralibus prope suturam intercalantibus. 



Variat quoque ad basim, striis spiralibus vix monstrantibus. 

 rugulis usque ad peripheriam cequalitcr radiantibus. 



In the form of the base and aperture and the wrinkles of 

 growth, this species resembles Solarium : in its two prominent 

 keels and scarcely united whirls it lias relations with Bifrontia, 

 (Omalaxis, H. Sf A. Ad. Gen. i. 214,) first discovered in a living 

 state by the indefatigable zeal of E.. M'Andrew, Esq. The 

 medial keel does not project beyond the upper and lower 

 ridges : the posterior keel is between the lower keel and the 

 sutirre. The adolescent portion, which is as usual nearly 

 smooth, though angulated at the spire, is almost concealed by 

 the penultimate portion ; the last whirl ho\\erer descends as in 

 Delphinula, only touching by the anterior keel. The base is 

 normally ornamented with two spiral lines, decussated in 

 squares between ; with rarer decussating lines ou the outer por- 



