41 



MONOGRAPH OF THE GENUS 

 PTEROCERAS. 



Char. Gen. Testa ovali, angulata, spiraliter costata ; spira 

 conica, anfractibus angulatis apertura elongata, antice que pos- 

 tice, in digitam canaliferam desinente ; interne longitudinaliter 

 uniplicata; labio externo setate dilatato, incrasato, digitis 

 arcuatis canaliferis diviso ; operculo corneo, ungulatim 

 acuminato. 



The shells of this genus are generally of an oval shape, 

 angulated on the upper part of the whorls, and spirally ribbed. 

 The aperture is elongated, terminating at each extremity in 

 a lengthened caudiform canal. When full grown, the outer 

 lip is expanded, thickened and divided into from four to eight 

 hollow digitations. The anterior sinus is separated from the 

 canal by a prominent lobe ; the inner lip is thickened, and, 

 when mature, unites with the upper part of the outer lip in 

 partly concealing the spire. 



DESCRIPTIONS. 



1. Pt. truncatum. (pi. xi. f. 13.) Lam. Anim. sans Vert, 

 vii. p. 195. 



T. magna, ovali, spiraliter costata, alba, lutescente, vel 

 pallide rosea ; spira nodulosa, apice truncata, complanata; 

 apertura kevi; labio externo digitis quinquebrevibus,sub-rectis. 



This species, which is the largest of the genus, differs from 

 P. Lambis in the apex of the spire, the top of which is trun- 

 cated and flattened. The shell is about a foot in length, and 

 in the oldest specimens we have seen, the digitations are much 

 shorter than those of P. Lambis. Polynesia and Australia. 



2. Pt. Lambis. (pi. xi. f. 5, 6, 7.) Lin. Syst. Nat. p. 1208. 

 T. subtrigona, spiraliter costata, angulata ; anfractu ultimo 



tuberculorum seriebus tribus ; tuberculo magno dorsali ad 

 angulum ; apertura lsevi, canalibus sub-elongatis ; labio 

 externo expanso ; digitis quinque sub-recurvis. 



This species seldom reaches above half the length of P. 

 truncatus, and the apex of the spire is acute. The colour 

 varies in the aperture from white to dark clouded orange, and 



E 



