928 MOLLUSCA AND BKACHIOPODA 



There is not by any means the same amount of diversity in the 

 structure of the shell in the class of Gastropods, a certain typical 

 plan of construction being common to by far the greater number of 

 them. The small proportion of animal matter contained in most of 

 these shells is a very marked feature in their character, and it 

 serves to render other features indistinct, since the residuum left 

 after the removal of the calcareous matter is usually so imperfect as 

 to give no clue whatever to the explanation of the appearances shown 

 by sections. Nevertheless, the structure of these shells is by no 

 means homogeneous, but always exhibits indications, more or less 

 clear, of a definite arrangement. The * porcellanous ' shells are com- 

 posed of three layers, all presenting the same kind of structure, but 

 each differing from the others in the mode in which this is disposed. 

 For each layer is made up of an assemblage of thin laminae placed 

 side by side, which separate one from another, apparently in the 

 planes of rhomboidal cleavage, when the shell is fractured ; and, as 

 was first pointed out by Mr. Bowerbank, each of these laminae con- 

 sists of a series of elongated spicules (considered by him as prismatic 

 cells filled with carbonate of lime) lying side by side in close apposi- 

 tion ; and these series are disposed alternately in contrary directions, 

 so as to intersect each other nearly at right angles, though still 

 lying in parallel planes. The direction of the planes is different, 

 however, in the three layers of the shell, bearing the same relation 

 to each other as have those three sides of a cube which meet each 

 other at the same angle ; and by this arrangement, which is better 

 seen in the fractured edge of the Cyprcea or any similar shell than 

 in thin sections, the strength of the shell is greatly augmented. A 

 similar arrangement, obviously answering the same purpose, has 

 been shown by the late Sir John Tomes to exist in the enamel 

 of the teeth of Rodentia, and by Professor Rolleston in that of the 

 elephant. 



The principal departures from this plan of structure are seen in 

 Patella, Chiton, Haliotis, Turbo and its allies, and in the ' naked ' 

 Gastropods, many of which last, both terrestrial and marine, have 

 some rudiment of a shell. Thus in the common slug, Limax rufus, 

 a thin oval plate of calcareous texture is found imbedded in the 

 shield-like fold of the mantle covering the fore part of its back ; and 

 if this be examined in an early stage of its growth it is found to 

 consist of an aggregation of minute calcareous nodules, generally 

 somewhat hexagonal in form, and sometimes quite transparent, 

 whilst in other instances it presents an appearance closely resembling 

 that delineated in fig. 698. In the epidermis of the mantle of some 

 species of Doris, on the other hand, we find long calcareous spicules, 

 generally lying in parallel directions, but not in contact with each 

 other, giving firmness to the whole of its dorsal portion ; and these 

 are sometimes covered with small tubercles, like the spicules of 



British Fossil Brachiopoda, published by the Palseontographical Society. A very 

 remarkable example of the importance of the presence or absence of the perforations 

 in distinguishing shells whose internal structure shows them to be generically dif- 

 ferent, whilst from their external conformation they would be supposed to be not 

 only generically but specifically identical, will be found in the Ann. Nat. Hist. 

 ser. iii. vol. xx. 1867, p. 68. 



