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and Bulinus. The whole of these agree in the nature of their struc- 

 ture in the most perfect manner, with the exception of a mode of 

 arrangement which I shall presently describe. Thin sections of uni- 

 valve shells, made by the lapidary, and examined by transmitted 

 light, afforded so little information that I had immediate recourse 

 to the aid of the Lieberkuhn ; and upon examining in this manner 

 surfaces fractured at right angles to the outer and inner planes of the 

 shell, and either parallel or at right angles to the lines of growth, I 

 obtained a very distinct and satisfactory view of the organic structures 

 and their modes of arrangement. I will not particularize the results 

 of the examination of the whole of the thirteen species operated 

 upon, but select for description Cypraa mauritiana, which, from the 

 great quantity of colouring matter intermixed with its structures, is 

 the best adapted for examination by the mode I adopted. Upon 

 submitting to examination a surface of this shell, fractured at right 

 angles to the lines of growth, it was found to consist of three distinct 

 strata, uniform in the nature of their structure, but alternating in the 

 mode of their disposition. Each stratum is formed of innumerable 

 plates, composed of elongated, prismatic, cellular structure ; each 

 plate consisting of a single series of cells parallel to each other. 

 These plates of cellular structure are disposed 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 22 degrees with the upper and 

 under surfaces of the shell, and present the appearance represented in 

 PI. xv. fig. 1 , a, as seen with a power of 280 linear, the portion re- 

 presented being the central and outer layers 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 always 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 pris- 

 matic cells, which are disposed at right angles to those I have just 

 described, or view them in an edgewise and endwise direction, they 

 present an appearance remarkably different 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 regularly 

 piled upon each other. This arises from the line of fracture passing 



