STRUCTURE OF SHELL OF GASTEROPODA. 515 



discrimination of the fossil shells belonging to these two groups 

 respectively. Great caution is necessary, however, in applying 

 this test; mere surface-markings cannot be relied on; and no 

 statement on this point is worthy of reliance, which is not based 

 on a microscopic examination of thin sections of the shell. In 

 the families Spiriferidce and Strophonemidce, on the other hand, 

 some species possess the perforations, whilst others are destitute 

 of them ; so that their presence or absence there only serves to 

 mark out subordinate groups. This, however, is what holds 

 good in regard to characters of almost every description, in other 

 departments of Natural History, as well as in this ; a character 

 which is of fundamental importance from its close relation to the 

 general plan of organization in one group, being, from its want 

 of constancy, of far less account in another.' 



342. There is not by any means the same amount of diversity 

 in the structure of the shell in the class of Gasteropoda, as that 

 which exists among the several tribes of Conchifera ; a certain 

 typical plan of construction being common to by far the greater 

 number of them. The small proportion of animal matter con- 

 tained 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 explana- 

 tion 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 an original 

 organic arrangement. The " porcellanous" shells are composed 

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

 each differing from the others in the mode in which this is dis- 

 posed. 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 consists of a series of elongated spicules 

 (considered by him as prismatic cells filled with carbonate of 

 lime) lying side by side in close apposition ; 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 Gyproea or any similar shell, than in thin 

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

 similar arrangement, obviously designed with the same purpose, 

 has been shown by Mr. Tomes to exist in the enamel of the 



' For a particular account of the Author's researches on this group, see his memoir 

 on the subject, forming part of the Introduction of Mr Davidson's " Monograph of the 

 British Fossil Brachiopoda," published by the PalEeontographical Society. 



