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the former existence of a wound. It is difficult to divine the precise 

 office which they fulfil in the restoration of the wounded tissues, but 

 that they are connected with this process is evident from their being fre- 

 quently to be found amid the newly-formed layers of tessellated cells, 

 which are seen tier above tier, advancing from the healthy margin to- 

 wards the centre of the wound ; and, under favorable circumstances, 

 their communications may be traced with those permeating the old 

 periostracum. 



There are other tissues in bivalve shells, which, like the structure 

 of the periostracum just described, evince a much higher degree of 

 organization than naturalists have hitherto believed to exist in shells 

 of molluscous or conchiferous animals. Thus, in the inner surface 

 of many bivalves, there are tissues which very closely approach in 

 their structure to the areolar tissue, described by Mr. Bowman in his 

 treatise on mucous membrane, in the ' Cyclopaedia of Anatomy and 

 Physiology.'* In many shells the tissues, which I believe to be ana- 

 logous to those described by Mr. Bowman, are of extreme tenuity, 

 while in others they are much more easily to be observed ; but in 

 none which I have examined are they developed in a greater degree 

 of perfection than in Venus decussata, and to this therefore 1 shall 

 limit my description. If the shell be macerated in very dilute hydro- 

 chloric acid until it is entirely deprived of its calcareous matter, and 

 then examined in water, with the inner surface towards the eye, with 

 a power of 160 linear, we shall observe, immediately beneath the 

 membrane lining the interior of the shell, three or four layers of a 



* The author, in describing the areolar tissue, says, " It in truth consists of two 

 tissues, distinct from each other, and respectively allied to the white and to the yellow 

 fibrous tissues. The white fibrous element of areolar tissue is chiefly in the form of 

 bands of very unequal thickness, in which are to be seen numerous streaks taking the 

 general direction of the whole, but not parallel to the border, nor continuous from end 

 to end. These streaks more resemble the creases of a longitudinal folding tban inter- 

 vals between the separate fibrillae, for which they have been mistaken. These bands 

 split up without difficulty in the long direction, whence result fibrils of the most va- 

 ried width, the finest being far too minute for measurement, even with the best instru- 

 ments. These bands interlace and cross one another in various directions, and their 

 natural course is wavy. They frequently subdivide and join those near them. Be- 

 sides these bands, commonly called fasciculi, there are some finer filaments of the 

 utmost tenuity, which seem to take an uncertain course among the rest. The yellow 

 fibrous element is everywhere in the form of solitary fibrilla;, which correspond in 

 their essential characters with the tissue of that name. They are disposed to curl, 

 and are truly branched at intervals of variable length ; these branches (which usually 

 retain the size of the fibril from which they spring) becoming continuous with others 

 in the neighbourhood." 



