362 ' MAlfUAL OF THE MOLLTJSCA. 



in acid tlie free border [of the mantle] wliicli projects beyond tbe 

 marginal fold, and which, is applied to the extreme edge of the 

 shell, can be examined with advantage. The pallial cceca are 

 then completely exposed appended to the membrane in various 

 stages of development, and the spaces between them are found 

 studded all over with ratiier large, clear, oval, cell-like spots, which 

 are arranged with considerable regularity in rows, so that those 

 in the approximate rows alternate. These spots apparently 

 correspond to the bases of the prismatic columns of the shell ; 

 and if it be allowed that they represent spaces in which cal- 

 careous granules had been accumulated, it is easy to understand 

 how the fibrous or columnar structure is formed. A succession 

 of layers of such accumulated granules deposited one after the 

 other would result in the peculiar shell formation of the 

 Brachiopoda." The extremities of the prisms are not visible 

 on the external surface, but in the young individual of some 

 species, as Terebratula caput-serpentis, there is a thin layer of 

 calcareous matter, which seems to show that in some Brachio- 

 pods the shell is composed of two layers of shell, having a 

 diflPerent structure, as in the case of the Conchifera. 



Of all moUusca the Brachiopoda enjoy the greatest range both 

 of climate, and depth, and time ; they are found in tropical 

 and polar seas, in pools left by the ebbing tide, and at the 

 greatest depths hitherto explored by the dredge. At present 

 only 84 recent species are known ; but many more will probably 

 be found in the deep sea, which these shells mostly inhabit. 

 The number of living species is already greater than has been 

 discovered in any secondary stratum, but the vast abundance of 

 fossil specimens has made them seem more important than the living 

 types, which are still rare in the cabinets of collectors, though 

 far from being so in the sea. Above 1,800 extinct species of 

 Brachiopoda have been described, of which more than half are 

 found in England. They are distributed throughout all the 

 sedimentary rocks of marine origin from the Cambrian strata 

 upwards, and appear to have attained their maximum of specific 

 development in the Silurian age.* Some species {\!ik.Q Atrypa 

 reticularis) extend through a whole "system" of rocks, and 

 abound equally in both hemispheres; others (like Spirifera 

 striata) range from the Cordillera to the Ural mountains. One 

 recent Terebratula [caput-serpentis) made its appearance in the 

 Miocene Tertiary ; whilst others, scarcely distinguishable from 



* The number of Silurian species amounts to 690 ; but these were not all living at 

 one time, they were obtained from a whole series of deposits, representing a succession 

 gf jieriodfl. 



