TRANSACTIONS OF SECTION C. 701 



In No. 2 the pure crystallised minerals were employed, and in No. 3 powdered 

 shells. 



Result. — About equal quantities of the two substances were dissolved. 



Conclusion. — That the instahility of Aragonite shells, as well as tbeir opacity, is 

 due not directly to their mineralogical constitution, but to their structure. 



The geological conditions favourable to the removal of Aragonite shells is found 

 to be — 



a. Enclosure in peruieable beds. 



h. Floio of carbonated water. 



Part II. — An account of the worlc clone in following out the foregoin;] observa- 

 tions, and in the e.iriminaii.on of certain organisms belonging to groups not yet 

 classified according to their mineralogical constitution. 



The constitution of the shells, &c., mentioned hereafter, was ascertained by 

 determination of the sp. grs. 



The observations were made in following out indications obtained — 



(1) From the known inferior stability of Aragonite shells. 



(2) From the rule which appeared to hold with regard to the translucency O'f 

 Calcite fossils and the opacity of those of Aragonite. 



Gasteropoda.— Scalaria (fossil) sp. gr. 2 685 Calcite. Mure.r tortuosus (fossil) 

 has thick opaque inner layer. 



Sp. gr. 2-85 ; tlierefore probably mainly Aragonite. From a comparison of the 

 Calcite layers of this shell and of Purpura lapillus, we are led to regard the crag- 

 Pm-pura tetragona as a variety of Mure.v erinaceus. 



Tectura testudinnria (recent) sp. gr. 2-8.34. Fossil tecturre have an opaque inner 

 layer; therefore jirobably Aragonite. 



Fusus.—1h& determination of three species confirmed the opinion expressed by 

 one of us in the paper before cited. 



F. antiqnus, sp. gr. 2 6G8 Calcite outer layer. 

 F. costifcr „ 2-88 Aragonite. 

 1<\ pyriformis „ 2-9o Aragonite. 

 /-'. longcevus „ 2-89 Aragonite. 



Cephalopoda. — Ammonites, from their appearance and mode of occurrence, are 

 probably to be regarded as Aragonite, but the aptychi which are found well pre- 

 served with casts of Ammonites are translucent, and their sp. gr., 2-70, proves that 

 they are Calcite. 



Belem7iites.~Th.e guard has a sp. gr. of 2-67, and is Calcite. The phragmacone 

 is not preserved in porous beds, and is opaque. The sp. gr. of a specimen infiltered, 

 but not replaced by Calcite, was 2-75 ; we therefore consider it to be Aragonite. 



Placophora. — C'/if'i^on (recent), sp. gr. 2-848. Aragonite. 



Heteropoda. — Dolabella (recent), sp. gr. 2-859. Aragonite. 



Lamellibrunchiata. — Pecten opercularis (recent), sp. gr. 2-70. Calcite. Pectun- 

 culus glycimeris (recent), sp. gr. 2-845. Aragonite. Artemis lentiformis (fossil), 

 opaque sp. gr. 2 84. Aragonite. 



He.vacoralla. — All the corals examined by Dr. Sorby were mainlv or entirely 

 Aragonite. We examined one of tlie Upper Chalk corals, ParasmiUa centralis, 

 and found it to be translucent, and to have a sp. gr. of 2 7 ; therefore it is Calcite. 



Polyzoa. — Dr. Sorby found many forms to be composed of Calcite with Ara- 

 gonite, and supposed that the two substances were mi.ved; but the observations of 

 one of us point to the conclusion that there is an outer layer of Aragonite. 



Foraminifera.— Dr. Sorby classes these in his Calcite division ; but we are led 

 to believe that the Porcellanea are Aragonite. They are opaque in the fossil state, 

 and, so far as we can ascertain, are not found in beds from which the Vrao-onite 

 shells have been dissolved. " 



In Dixon's ' Geol. of Sussex,' 93 species and varieties of Foraminifera are re- 

 corded from the Chalk, and only one Porcellanous form is mentioned, and that 

 without the specific name. Experiments upon the comparative solubility of the 

 Porcellanea and Vitria confirm our impression. 



Teredo Norvegica.— Teredo is regarded by Dr. Sorby as a tvpical Calcite shell ; 



