564 



SHELL. 



their length. A great variety of appearances 

 is presented by this structure, according to 

 the direction in which it happens to be tra- 

 versed by the section ; but they are all in- 

 dicative of its peculiar character, which is 

 readily recognisable even in the minutest 

 fragment, although its nature yet remains 

 doubtful. The cells, if cells they be, must be 

 excessively flattened ; and no vestige of them 

 can be traced in the decalcified shell ; whilst, 

 on the other hand, the membranous residuum 

 does not give any distinct indication of having 

 been plicated with the regularity necessary to 

 produce such a remarkable appearance. When 

 any recent species of Terebratula is examined, 

 save Ter. psittacea (which is now generally 

 excluded from the genus on other grounds), 

 an additional peculiarity is observed ; con- 

 sisting of the presence of a large number of 

 perforations in the shell, generally passing 

 obliquely from one surface to the other, and 

 terminating internally by an open orifice (Jig. 

 418.), whilst on the exterior they are covered 

 in by the periostracum. Their diameter, 

 which is greatest towards the external surface, 

 varies in different species from about 1- 1800th 

 to 1 -500th of an inch ; and there is a con- 

 siderable difference, also, in their degree of 

 proximity to each other. In some fossil spe- 

 cies, as Ter. bul'ata, the interval between the 

 passages is scarcely greater than the diameter 

 of the passages themselves. When a portion 

 of one of these shells, which has been pre- 

 served with the animal in spirit, has been 

 completely decalcified by the action of dilute 

 acid, the membranous residuum presents a 

 very remarkable structure, no vestige of which 

 is seen in the ordinary bivalves. Attached to 

 the membranous films are a series of tubular 

 appendages, corresponding in diameter to the 

 perforations of the shell, and arranged at the 

 same distances C/?g. 419.) : the free extremi- 



iff. 419. 



Decalcified membrane of shell of Terebratula australis, 

 showing the ccecal tubuli, which occupy the perfora- 

 tions of the shell : tlie tubuli are filled with minute 

 cells. Magnified 150 diameters. 



ties of these appendages have distinct ccecal 

 terminations ; and when a sufficient magnify- 

 ing power is employed, it is found that their 



contents are distinctly cellular, resembling the 

 cells in the interior of glandular follicles. 

 These coecal tubuli lie in the perforations of 

 the shell, and open on its inner surface ; but 

 there does not appear to be any system of 

 tubes or canals for collecting the matter 

 poured out from them, each ccecum having 

 its distinct and independent termination on 

 the internal surface of the shell. The surface 

 of the mantle in contact with the shell is 

 found to be studded with minute cells, cor- 

 responding in size and aspect with those con- 

 tained in the ccecal tubuli. The physiological 

 purpose of this curious structure is at present 

 a mystery ; but there can be little doubt that 

 it is a very important one in the economy of 

 the animal, when we see the shell thus ren- 

 dered subservient to the special protection of 

 the ccecal appendages. The perforations are 

 wanting in a large proportion of the very 

 numerous species of fossil Terebratulce ; and 

 there would appear strong reason for regard- 

 ing their presence or absence as a character of 

 fundamental importance in the subdivision of 

 this important genus.* In most of the non- 

 perforated species, the shell is readily divisible 

 into thin micaceous plates, which exhibit the 

 characteristic texture of the shell in great 

 perfection ; and as this texture undergoes 

 remarkably little change in the act of fossil- 

 isation, it is often possible to recognise a Tere- 

 bratula from a very minute fragment, imbedded 

 even in the palaeozoic strata. A very similar 

 structure exists in several genera allied to 

 Terebratula ; and in some of these, also, as 

 Orthis and Spi?'ifer, the distinction has to be 

 established between the perforated and non- 

 perforated species ; whilst in Atrypa (to which 

 the recent Ter. psittacea properly belongs), all 

 the species are destitute of perforations. 



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 con- 

 struction 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 cha- 

 racter ; 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 sec- 

 tions. Nevertheless, the structure of these 

 shells is by no means homogeneous, but al- 

 ways exhibits indications, more or less clear, 

 of an original organic arrangement. The 

 porcellanous shells, as formerly stated (vol. ii. 

 p. 384), are composed of three layers, all pre- 

 senting the same kind of structure, but each 

 differing from the others in the mode in which 

 this is arranged. This structure was de- 

 scribed by Mr. Gra} -f- as the result of rhom- 

 boidal crystallisation ; each layer being com- 



* See a Paper on the Subdivision of the Genus 

 Terebratula, by Mr. J. Morris, in the Journal of the 

 Geological Society, vol. ii. p. 382. 



t Phil. Trans. 1833, p. 790. 



