8&2 MANUAL OF THE MOLLUSCA. 
in acid the free border [of the mantle] which projects beyond the 
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 yarious 
stages of development, and the spaces between them are found 
studded all over withrather 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 T'erebratula caput-serpentis, there is a thin layer of 
calcareous matter, which seems to show that in some Brachio- 
pods the sheil is composed of two layers of shell, haying a 
different structure, as in the case of the Conchifera. 
Of all mollusca 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 importantthan 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 (like 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 
‘f periods. 
