MULTILOCULAR SHELLS. 257 



The chambered cone, which is called the alveolus, is not only 

 enveloped by this radiated appendage, but has a thin shelly coating 

 of its own, concealing the edges of the septa and the chambers. 

 Towards the mouth, this is the only covering of the alveolus; for the 

 radiated envelope grows thinner and thinner, in proportion as the 

 cone enlarges, till the latter becomes fully exposed. Such at least is 

 the case in the fossil shell ; in which also the inner covering of the 

 alveolus is often found decomposed, so that the edges of the cups 

 and the chambers appear. The latter are generally extremely small, 

 their breadth being less than the thickness of the cup on either side. 



The alveolus is often highly pyritous, the septa appearing like 

 golden cups, and the shelly coating having the same splendid lustre; 

 which is never the case with the radiated part of the fossil. Yet the 

 latter is the most firm and durable part, for it is generally entire, 

 while the former is more or less mutilated, often compressed, and 

 very frequently altogether lost. The alveolus, in many instances, is 

 found detached from its radiated sheath ; though this does not occur 

 so often in our strata, as in some other countries. In this detached 

 state, the alveolus has been viewed as a shell of another oenus 

 called ORTHOCERA, a genus which we have no hesitation in rejecting. 

 Mr. Sowerby, who adopts the term as comprehending the belemnite 

 family, judiciously intimates, that the orthocerae which are considered 

 as too large to belong to any known belemnite, may have had a thin- 

 ner and shorter envelope which has been lost. Some of our alveoli 

 expand to such a size, as to seem greatly too large for their belemnitic 

 terminations, which only cover about half their length. 



No shell is more extensively diffused in our rocks than the 

 belemnite. It occurs in all our strata, from the upper chalk to the 

 lowest shale, except the siliceous sandstone, the sandy and bituminous 

 shale, and the coal. It is most abundant in the aluminous strata, the 

 upper shale, and the calcareous sandstone. 



3 R 



