ERAcnioroDA. 361 



it consists of flattened prisms of consicIeraLlo Icngtli, arranged 

 parallel to each other with great regularitj':, and obliquely 

 to the surfaces of the shell, the interior of which is imbricated 

 by their out-crop (Fig. 146). This struc- 

 ture is found only in the IlhynchoneUidce ; 

 but in most — perhaps all the other 

 Brachiopoda* — the shell is traversed by 

 canals from one surface to the other, 

 nearly vertically, and regularly, the dis- 

 tance and size of the perforations varying 

 with the species. Their external orifices 

 are trumpet-shaped, the inner often very 

 small ; sometimes they bifurcate towards ^^- ' ' (■ " u a. 

 the exterior, and in CVania they become aborescent. The canals 

 are occupied by coecal processes of the outer mantle-layer,t 

 and are covered externally by a thickening of the cj)idermis. 

 Mr. Huxley has suggested that these cceca are analogous to 

 the vascular processes by which in many ascidians the ttmic 

 adheres to the test; the extent of which adhesion varies in 

 closely allied genera. The large tubular spines of tho Froduc- 

 tidce must have been also lined by j^rolongations of the mantle ; 

 but their development was more probably related to the main- 

 tenance of the shell in a fixed position, than to the internal 

 economy of the animal. (King.) Dr. Carpenter states that 

 the shell of the Dmchiojpoda generally contains less animal 

 matter than other bivalyes ; but that Dis'cina and Lingula con- 

 sist almost entirely of a horny animal substance, which is 

 laminar, and penetrated by oblique tubuli of extreme minute- 

 ness. He has also shown that there is not in these shells that 

 distinction between the outer and inner layers, cither in struc- 

 ture or mode of growth, which prevails among the ordinary 

 bivalves ; the inner layers only differ in tho minute size of tho 

 perforations, and the whole thickness corresponds with the 

 outer layer only in the LarncUihranchiata. The loop, or 

 brachial processes, are always impunctate. Mr. Hancock's 

 researches would tend to show that these conclusions arc gene- 

 rally correct, but not entirely so. " When the shell is dissolved 



* The fossil sliells of the older rocks are so generally pseudomorphous, or partake of 

 the metaniorphic character of the rock itself, that it is dilKcult to obtain specimens la 

 A state fit for microscopic examination. 



t Called the"liiiinf^ membrane of the shell," by Dr. Carpenter, (Davidson Intr. 

 Mon. Bracli.) M. Qucckctt states that tlie perforations are closed externally by disks, 

 surrounded by radiating lines, supposed to indicate the existence of vibratUe cilia in 

 tho living specimens. 



