70 Cornish and Kendall—Calcareous Organisms. 
to the translucency of calcite fossils and the opacity of those of 
aragonite. 
Nature of Specimen. Weight of Specimen. Sp. gr. 
Crystal of aragonite hexagonal (twin form)............ 37°3560 grams ... 2°933 
Avragonite, fibrous variety ja-nveseseceeseceseccssonscess 23°5111_ ,, wee =2°857 
Artemis lentiformis (fossil, opaque) ..........sceeeeeeees 4°7490  ,, ... 2 840 
Pectunculus glycimeris (TeCENt) ........ececenecececeeeeees 3°9445,, ws (2°840 
Calcite (cleavage rhombohedron) ...............2ceeeeeee S022) hes, see o2rallO 
Pecten opercularis (fossil, translucent) ...............068 18 (Mle Bodh. RAW) 
Pecten opercularis, (LECENt)  .......secesecasserecnseoosee Tey, 5 ve 2°70 
GASTEROPODA. 
Scalaria, to which reference was made in the paper before quoted, 
proves to be calcite, as suggested from its translucency and mode of 
occurrence in the Coralline Crag. Its specific gravity is 2-685. 
Murex tortuosus has a thick opaque inner layer, while the invest- 
ment constituting the frills or varices is translucent. The specific 
gravity 2°85 indicates that the greater part of the shell is aragonite, 
as suggested by its appearance. 
We are disposed to regard these features as valuable in the deter- 
mination of the true affinities of the so-called Purpura tetragona, 
regarding which the late Dr. Jeffreys and Mr. Searles V. Wood jun., 
were at issue. Purpura as typified by P. /apillus has an extremely 
thin opaque layer, which does not reach the edge of the outer lip, 
and the specific gravity corresponds with that of calcite shells. 
P. tetragona on the other hand is opaque, with a translucent layer 
so insignificant in thickness that it is removed by attrition from all 
the prominences. The translucent layer is not visible within the 
outer lip. 
The affinities of P. tetragona are therefore probably with Murex 
arenaceus rather than with P. lapillus. 
Tectura testudinaria. Sp. gr. of recent specimen 2°834. In the 
fossil state it is opaque, with a translucent outer layer, hence we 
may conclude that, as in the case of M. tortuosus, the inner layer is 
aragonite and the outer calcite. 
Fusvs. 
This genus furnishes evidence, which we consider to be conclusive, 
respecting the determination of the constitution of calcareous shells 
by their opacity or translucency in the fossil state. 
Mr. Sorby stated in his address that the inner layer of usus, as 
typified by F’. antiquus, is aragonite, the outer calcite,.and in the 
paper by one of us already cited it was mentioned that the inner 
layer was opaque and the outer translucent, and several fossil 
species were instanced, which, being entirely opaque, were probably 
wholly aragonite. The sp. gr. of three species which have been 
determined by us shows them to be wholly aragonite. We give 
the sp. gr. of F. antiquus for comparison. 
F. ANUIQUuus vo ccccccerceceeee . sp. gr. 2°668 (very thin aragonite layer) 
TH GOSBOICP * posanveeeonboaca0l A 2°88 
LOS SIPGTED MOOS: Gadonouebescen AA 2°95 
Fr. longevus ...... po0eednab080 ap 2°89 
