THE OPINIOX OF BOWERBANK. ;389 



the interesting researches of Mr. Bowerbank and Dr. Car- 

 penter. 



After a careful microscopical examination of many species 

 in various families of Mollusca, Mr. Bowerbank concludes 

 that the shell is highly organized. The shell of the pectini- 

 branchial and pulmonatcd Mollusks consists, when perfected, 

 of three distinct strata uniform in the nature of their struc- 

 ture, but alternating in the mode of disposition of their 

 calcareous matter. " Each stratum is formed of innume- 

 rable plates composed of elongated, prismatic, cellular struc- 

 ture ; each plate consisting of a single series of cells parallel 

 to each other. These plates of cellular structure are dis- 

 posed alternately in contrary directions, so that each series 

 of cells intersects the one beneath it at nearly right angles, 

 and the whole of them are so disposed that the axes of the 

 cells form angles of about twenty-two degrees with the 

 upper and under surfaces of the shelL" " The planes of 

 these plates of prismatic cellular structure, are always either 

 as nearly as possible parallel or at right angles to the lines 

 of growth, and the mode of arrangement is invariable in 

 each separate stratum, and ahvays opposed to that of the 

 stratum either above or below it ; so that if the plates of 

 cells in the central stratum be parallel to the lines of growth 

 in the shell, those of the outer and inner strata are at right 

 angles to them. When we view the stratum of plates of 

 prismatic cells, which are disposed at right angles to those 

 I have just described, or view them in an edgewise and end- 

 wise direction, they present an appearance remarkably dif- 

 ferent from that which we have previously described. In 

 this case the whole of the stratum seems to be composed of 

 a series of basaltiform columns, each column having the 

 appearance of being formed of a series of single cells regu- 

 larly piled upon each other. This arises from the line of 

 fracture passing in a direction about equally oblique to the 

 planes of the whole of the plates of prismatic cells. From 

 this description of the alternating mode of arrangement of 

 the plates of cells, it will be obvious that if, on examining a 

 shell, we find a section parallel to the lines of growth, with 

 the plates of the central stratum agreeing in their direction 

 with those lines, while those of the outer and inner ones are 

 at right angles to them, we shall find the apparent direction 

 of the plates exactly the reverse in each stratum if we view 

 a second section of the same shell at right angles to the 

 lines of growth. These modes of arrangement of the tis- 

 sues are invariable in the same species and genus, as far as 

 I have had an opportunity of judging ; and it is somewhat 



