62 MICROSCOPIC OBJECTS. 



tions depolarise light, so that the shell appears bright upon 

 a dark ground in the polarising microscope ; hence it 

 follows that the calcareous matter of the shell is in a 

 state of crystalluie aggregation. 



Dr. Carpenter classifies shells into — 1 . Prismatic cel- 

 lular structure, as exemplified in the Pinnce; — 2. Mem- 

 branous shell substance — examples, Mya, Anatina and 

 Thracia ; — 3. Nacreous or pearl structure — example, the 

 inner substance of certain species of the Ostrea and My- 

 tilus ; — 4. Tubular structure — example, outer layer of 

 Anomia Ephippium, Lima scahra, and in Chama florida; 

 in some cases the tubes run at a distance Jfrom each 

 other obhquely through the shell, as in Area Noee ; — 

 5. Cancellated structure. Examples of this latter division 

 which somewhat resemble the cancelli of bone, are only 

 met with in certain fossil shells belonging to the Rudistes 

 — example, Pleurorhynchus Hibernicus. 



Shell should be examined microscopically in three ways 

 — ^by reflected light, by ordinary transmitted rays, and by 

 transmitted polarised hght. For the first, fragments of 

 shell will suffice ; for the second, thin sections cut both 

 vertically and transversely are necessary. To exhibit the 

 animal basis of shell, specimens must be procured in 

 which the earthy matter has been removed by immersion 

 in dilute hydrochloric acid. — Magnifying powers from 

 10 to 250 diameters. 



Shell of the Crab. — If a portion of the shell of a crab, 

 about half an inch in diameter, be ground very thin, so 

 as to leave only the outer layer of colouring- cells, and 



