306 BRACHIOPODA. 
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 brachiopods the shell is composed 
of two layers of shell, having a different structure, as in the case 
of the Conchifera. 
The Lamp-shells are all natives of the sea. They are found 
hanging from the branches of corals, the under sides of shelving 
rocks, and the cavities of other shells. Specimens obtained 
from rocky situations are frequently distorted, and those from 
stony and gravelly beds, where there is motion in the waters, 
have the beak worn, the foramen large, and the ornamental 
sculpturing of the valves less sharply finished. On clay beds, 
as in the deep clay strata, they are seldom found ; but where the 
bottom consists of calcareous mud they appear to be very 
abundant, mooring themselves to every daa substance on the 
sea-bed, and clustering one upon the other. 
Of all mollusea 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 compara- 
tively few recent species are known; but many more will probably 
be found by dredging 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 abund- 
ance 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 4000 
extinct species of Brachiopoda have been described, of which 
a large proportion are found in Europe. 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 Spurifera 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 it, are found in the Upper Oolite and 
throughout the Chalk series and London Clay.—Woopwarp. 
