172 MISCELLANEOUS COMMUNICATIONS. 



ing matter of the red Gooseberries which Berzelius did not examine, he 

 knows not. The red colouring matter of the leaves, half precipitated by 

 limewater, gives a green precipitate, while the liquor acquires a paler red 

 tint. Thus this colouring matter can no longer be regarded as originally 



blue Berzelius, Opere Citato. 



Observations on the Formation of Shells. — Mr. Gray whose pa- 

 pers in the Philosophical Transactions are well known, lately delivered a 

 valuable Lecture on the Growth and Structure of Shells, at the Royal In- 

 stitution. He explained how Shell is secreted by a portion of the skin of 

 the animal termed the mantel, at the edge of which, termed the collar, the 

 process usually goes on with the greatest vigour. The simplest form of 

 shell known to us, is that of a more or less flattened cone, as exemplified in 

 the common limpet, and all other varieties of form may be shown to be mo- 

 difications of this type ; the whole structure of shells being caused by the 

 animal's growing most to one side, and thus turning round a spiral axis. 

 Shells are subject to periods of unusual increase, at which times strong bands 

 or ribs are secreted, which add materially to their strength ; the causes of 

 this phenomenon are however unknown. In many shells we find large 

 spines ; these at one time corresponded to certain processes of the body of the 

 animal. But when a shell continues to increase in size, and to turn round 

 its axis, these spines might form a serious obstacle to its further growth. To 

 provide, then, against this inconvenience, nature has bestowed on the animal 

 a power of removing such spines as may be in its way. Mr. Gray did not 

 explain how this is effected, we presume he means by some process of ab- 

 sorption, but we may remark, that the question as to the mode of this re- 

 moval, is not yet set at rest by naturalists. At different periods of their 

 growth, some shells vary so much in their appearance, that naturalists gene- 

 rally, even including Lamarck, have believed that the animals had the power 

 of casting their shells, like many of the Crustacea. This opinion has, how- 

 ever, been satisfactorily disproved : and the great change in their appearance 

 is caused by the animal's turning back the aloe of its mantel over the edges 

 of the shell, and secreting a fresh layer of shell, as in the case of the cowries. 

 The beautiful variety of colours in shells is produced by a number of glands 

 which secrete colouring matter, and are usually seated around the neck of 

 the mantel. If these glands secrete constantly, then the shell is marked by 

 coloured bands, but if they do so only at times, spots are produced ; and thus 

 every variety of colouring and marking may be explained. When the mi- 

 nute structure of shells is examined, it is found that the materials are in 

 many cases not deposited in parallel layers, but in a method which greatly 

 increases their mechanical strength, and which has been applied to the tim- 

 bering of ships. Some s'lells are provided with an external membrane, which 

 may be considered as analogous to the periosteum of bone, and has been 

 termed the periostracum while others are devoid of it. The animals inhabit- 

 ing shells are very generally provided with a flat round layer of bone situated 

 on the inferior surface of their bodies, with which they close the aperture of 

 their shells, when their bodies have been retracted within. These have been 

 termed opercula. Mr. Gray concluded by alluding hastily to the structure 

 of the bivalves, whose shells are constructed on the same plan as those of the 

 univalves; and by explaining the beautiful provision in their hinges, by 

 means of which the two valves are kept open by ligamentous bands when 



