390 BOWERBANK'S RESEARCHES : 



remarkable, that of the eight genera examined, four assumed 

 one mode of arrangement and four the other. Thus, upon 

 viewing sections parallel to the lines of growth of the shells, 

 I found that in Cypraea, Cassis, Ampullaria, and Bulinus, 

 the plates of the outer and inner stratum were parallel to 

 the lines of growth, while those of the central one were at 

 right angles to them. On the contrary, in Conus, Pyrula, 

 Oliva, and Voluta, the plates of the outer and inner stratum 

 were at right angles to the lines of growth, and those of 

 the central stratum parallel to them. And it is worthy of 

 remark, that the porcellanous shells do not agree in the 

 mode of arrangement of their tissues, as we find Cypraea 

 opposed to Conus, Oliva, and Voluta, and agreeing with 

 Cassis, Ampullaria, and Bulinus." 



A considerable deviation from this structure is met wdth 

 in the shells of the Mollusca referred by Milne-Edwards to 

 his order Opistobranches. These Mollusks approximate in 

 anatomical character to the bivalve class, and Mr. Bowerbank 

 has ascertained that the same inclination appears in the 

 structure of the shell, for this, in the arrangement of the 

 cells and other tissues, approaches nearly to the majority of 

 bivalves. In the univalve shells in question, of which Hali- 

 otis may be assumed to be the type, there is no oblique 

 disposition of prismatic cells, but a regular basaltiform 

 columnar arrangement of cells, whose lines of direction are 

 at right angles to the outer and inner surfaces of the shell. 

 Neither is the shell composed of three strata, but of several 

 layers, consisting of a series of membranes (whose cells are 

 filled with calcareous matter) adhering to each other, and 

 having their planes parallel to the surface. These layers are 

 likewise permeated with tortuous canals, analogous to the 

 Haver sian canals in bone. Like the Haversian canals in 

 the bones of the higher classes of animals, they anastomose 

 with each other, and especially in Ostrea; " their courses 

 are equally tortuous and irregular, and their parietes are both 

 lined with a mucous or gelatinous coat. It is true, the Ha- 

 versian canals in the bones of man are on an average 1 -500th 

 of an inch in diameter, while those of shells are 1 -5,000th 

 or 1 -6,000th of an inch; but their diminutive size in the shell 

 is not out of proportion when we consider the difference in 

 the size of the animals. There is another tissue apparent in 

 the structure of this shell, which draws still closer the alli- 

 ance which exists between it and bone. If we examine the 

 membranous remains by transmitted light, with a linear 

 power of 1,000, we find embedded in the spaces intervening 

 between the sections of the canals an abundance of the 



