92 THE MICROSCOPIST. 



lately attracted the attention of inicroscopists, but since the 

 year 1842 the subject has been scientifically investigated by 

 Mr. Bowerbank and Dr. Carpenter. According to the experi- 

 ments of the latter gentleman, undertaken at the request and 

 expense of the British Association, the calcareous matter in 

 all shells is nearly equally crystaline in its aggregation, and 

 the particular forms which their fracture presents are deter- 

 mined chiefly, though not entirely, by the arrangement of the 

 animal basis of the shell, which possesses a more or less highly- 

 organized structure. 



All thin sections of recent shell are translucent, except 

 when the coloring matter is opaque, or when the calcareous 

 matter is deposited in a chalky state between the true laminae 

 of the shell, as in the oyster. 



Dr. Carpenter classifies shells, into 1. Prismatic cellular 

 structure, as exemplified in the Pinnce. 2. Membranous shell 

 substance, as the My a, Anatina^ and Thracia. 3. Nacreous 

 or pearl structure, as the inner surface of some species of Ostrea 

 and Mytilus. 4. Tubular structure, as the outer layer of Ano- 

 mia Ephippium, Lima scabra, &c. In some cases the tubes 

 run at a distance from each other obliquely through the shell, 

 as in Area Noce. 5. Cancellated structure. Examples of this 

 latter division, which somewhat resemble the cancelli of bone, 

 are only met with in certain fossil shells. 



Shell should be examined microscopically in three ways : by 

 reflected, transmitted, and polarized light. For the first, frag- 

 ments of shell will suffice; for the others, thin sections, cut 

 both vertically and transversely, are necessary. To exhibit the 

 animal basis of shell, specimens may be treated in the manner 

 recommended for coral. 



SCALES OF FISH. M. Agassiz has arranged the class of 

 fishes into four orders, according to the structure of their 

 covering, as follows : 



