MAZATLAN UNIVALVES 247 



basi et uinbilico spiraliter liratis ; ad peripheriam carind acutd, 

 prominente, alterd spiram ascendente, minus prominente ; inter 

 eas suturd valde impressd ; aperturd anfr. penult, vix ad cari- 

 nam attingente. 



This very remarkable shell has the general appearance of 

 Helix (Iberus) G-ualterianus. It 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 denned, 

 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. "04 by "025, div. 180. 



Hob Mazatian ; 1 broken specimen off Spondylus calcifer ; 

 Upool Col. 



Tablet 1162 contains the specimen. 



312. ? VITRINELLA OKBIS, n. s. 



? V. t. discoided, diaphand, hyalind ; valde complanatd, 

 spird planatd, basi subplanatd ; ad peripheriam rotundatam, 

 et intus umbilicum apertissimum, striis spiralibus paucis, plus 

 minusve impresses ; infra et supra Icevi, interdum, stria spirali 

 juxta suturam Jiaud impressam ; anfractu ultimo penultimum 

 parum attingente ; aperturd angustd, spiraliter elongatd, sub- 

 guadratd ; labro sinuato, posticc producto. 



This singular species has relationship to Discohelix, Dlcr. 

 Palceont. i. 132, = Orbis, Lea; of which, as of Vitrinella, the 

 animal is unknown, being classed with Littorinidae by Phil. 

 Handb. Conch, p. 174, but with Architectonicidas by H. fy A. 

 Ad. Gen. i. 211. 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 Chamse. 



