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angular by compression, but the number of angles is by no means 

 certain, and they vary in this respect from three to seven or more. 



The reparation of wounded and diseased shells is a part of our sub- 

 ject which is of high interest to the physiologist, and upon which 

 much might be written, but I must on this occasion content myself 

 with but slightly adverting to it. 



In cases of fracture of the shell, the process of repair is exceed- 

 ingly similar to the analogous operations in the bones of the higher 

 classes of animals. An effusion of coagulable lymph takes place 

 cytoblasts are produced ; cellular structure quickly succeeds, in which 

 the earthy basis of the shell is secreted, and the fractured parts are 

 firmly cemented together. 



If the accident be of less extent, such as an injury inflicted on the 

 inner surface of the shell, the process is precisely that which I have 

 described as taking place in the extension of the lip in the young 

 shell of Helix aspersa. A new membrane is formed by the effusion 

 of coagulable lymph ; cytoblasts and cells are developed ; and secre- 

 tion of carbonate of lime commences at the edge of the membrane. 

 As the processes of development and secretion proceed gradually in- 

 wards, the centre of the membrane is eventually reached by the en- 

 croaching circle, and the whole of the wounded surface is covered by 

 a new stratum of cells and membranous tissue. Such precisely is the 

 operation which has taken place on the inner surface of one of the 

 shells of Modiola papuana which I have in my possession. 



The mode of effecting repairs in the periostracum is in some 

 respects different from the analogous operation in the shelly structure. 

 One of the specimens of Solen vagina in my possession has had this 

 organ pierced in a great number of places ; and the examination of 

 these wounds affords so many interesting phenomena, and exhibits 

 evidence of the existence of so high a degree of vitality, as to render 

 it necessary to enter somewhat at length into this part of our subject. 

 In this case the wounded part is in such a position as to preclude 

 the possibility of the animal exuding a layer of coagulable lymph so 

 as to form a new basement membrane, in the manner in which the 

 addition to the lip is effected in the univalve, or in the repair or ex- 

 tension of the inner surface of the bivalves. Nature has, therefore, 

 had recourse to other means for the production of the basement 

 membranes of the new tissues, which are required to be produced to 

 supply the places of those which have been destroyed or displaced ; 

 and this is effected by the slow extension of the inner layer of the 

 periostracum, which gradually encroaches from all sides on the 



