MAZATLAN UNIVALVES 47 
bast et umbilico spiraliter liratis ; ad peripheriam cariné acuta, 
prominente, alter& spiram ascendente, minus prominente ; mter 
eas suturd valde impressa ; apertura anfr. penult. vie ad cari- 
nan attingente. 
This very remarkable shell has the general appearance of 
Helix (Iberus) Gualterianus. Ji has however two keels, (that 
on the spire being less prominent) and a deep rather narrow 
umbilicus. The whirls rapidly increase in size both in breadth 
and thickness. The generic position is doubtful, the only 
specimen found having lost the greater portion of the last 
whirl. The general aspect and texture are like a very large 
Vitrinella, and the usual unformed portion is clearly defined, 
being much larger than in the typical species ; it differs how- 
ever in the comparative length of the adult part, which must 
have consisted of at least two complete whirls. The shell 
when perfect may have been ‘2 in length: the penultimate 
whirl measures long. ‘03, lat.*O4 by °025, div. 180°. 
Hab.—Mazatlan ; 1 broken specimen off Spondylus calcifer ; 
P’pool Col. 
Tablet 1162 contains the specimen. 
312. P VITRINELLA ORBIS, 2. S. 
?V. t. discoided, diaphand, hyalind ; valde complanata, 
spird planatd, basi subplanaid ; ad peripheriam rotundatam, 
et intus umbilicum apertissinum, striis spiralibus paueis, plus 
minusve impressis ; infra e& supra levi, interdum. strid spirali 
juxta suturam haud impressam ; anfractu ultimo penultimum 
parum attingente ; aperturd angustd, spiraliter elongata, sub- 
quadrata ; labro sinuato, postice producto. 
This singular species has relationship to Discohelix, Dkr. 
Paleont. i. 132,=Orbis, Lea; of which, as of Vitrinella, the 
animal is unknown, being classed with Littorinidee by PAzd. 
Handb. Conch. p. 174, but with Architectonicide by H. & A. 
Ad. Gen. i. 244. It has the appearance of the common flat 
Dundry Ammonites, with the periphery rounded and spirally 
striated. A beautifully perfect specimen, quite transparent, 
but probably immature, was found inhabiting the spiral portion 
of a dead Isapis maculosa, the mouth of which had been choked 
up with coralline, among the debris of a Spondylus. It 
measures long. ‘0075, lat. 033 by °027, div. 180°. Three 
other broken specimens were found on Chame. 
