Cornish and Kendall—Caleareous Organisms. ma. 
The exceptionally high sp. gr. of / pyriformis is no doubt to be 
accounted for by the fact that the specimen was impregnated with 
oxide of iron. 
CEPHALOPODA. 
Ammonites—From the mode of occurrence of these shells, the 
circumstance that in porous beds casts only are preserved, and the 
similarity in appearance of the shells to those of Mautilus (deter- 
mined by Mr. Sorby to be aragonite), we are justified in assigning 
them to the aragonite division. 
The Aptychi, however, have the translucency characteristic of 
calcite, and they are found well preserved in beds such as the Chalk, 
in which the Ammonites are only represented by casts. The 
determination of the sp. gr. 2°70 shows them to be calcite. 
Belemnites.—We find in Woodward’s Manual of Conchology, p. 1738, 
that according to the determination of Mr. Alex. Williams, the sp. gr. 
of the guard of Belemnites puzosianus is 2°674, and that of Belem- 
nitella mucronata 2-677; they are therefore calcite, as their trans- 
lucent appearance, familiar to all geologists, would lead one to 
suppose. 
The phragmacone of Belemnitella is not known, and as no aragonite 
shells are preserved in the Upper Chalk, we were led to believe that 
the phragmacones of Belemnites would prove to be aragonite. Mr. 
Robert Etheridge kindly furnished us with a specimen in which the 
chambers were filled with translucent calcite, while the septa and the 
siphuncle which were preserved presented the usual chalky appear- 
ance of aragonite. The specific gravity was found to be 2°75, 
justifying the belief that a small quantity of aragonite was present, 
with the greater certainty that the appearance of the specimen shows 
it to be free from oxide of iron, and the mode of deposition of the 
calcite in the chambers, together with its high degree of translucency, 
furnish guarantees of its purity. A specimen of B. minimus from 
the Gault has the phragmacone preserved, and it presents the 
appearance characteristic of aragonite. It may be of interest here to 
note that the homologous structure of sepia has been determined by 
Sorby to be aragonite. 
PoLYPLACOPHORA. 
Chiton (recent), sp. gr. 2°848, therefore aragonite. 
HETEROPODA. 
Dolabella (recent), sp. gr. 2°859, therefore aragonite. 
LAMELLIBRANCHIATA. 
Pecten opercularis (recent), sp. gr. 2°70, therefore calcite. 
Pectunculus glycimeris (recent), sp. gr. 2°845, therefore aragonite. 
Artemis lentiformis (fossil), sp. gr. 2°84, therefore aragonite. 
Teredo Norvegica.—Teredo is regarded by Dr. Sorby as a typical 
calcite shell: but certain tubular fossils found by one of us in the 
Crag, and which have been regarded as T. Norvegica, have the 
opacity characteristic of aragonite; and upon this circumstance and 
peculiarities in its mode of occurrence the opinion was based that the 
