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Updated: June 13, 2025
Thus the Cephalopoda are the most valuable, as having a more restricted range in time than the Gasteropoda; and these, again, are more characteristic of the particular stratigraphical subdivisions than are the Lamellibranchiate Bivalves, while these last, again, are more serviceable in classification than the Brachiopoda, a still lower class of shell-fish, which are the most enduring of all.
This genus of brachiopoda was first found in Silurian strata, and is exclusively a palaeozoic form. The name was derived from pente, five, and meros, a part, because both valves are divided by a central septum, making four chambers, and in one valve the septum itself contains a small chamber, making five.
Not only have the invertebrata, as shown by geological data, altered at a less rapid rate than the vertebrata, but if we take one of the classes of the former, as for example the mollusca, we find those of more simple structure to have varied at a slower rate than those of a higher and more complex organisation; the Brachiopoda, for example, more slowly than the lamellibranchiate bivalves, while the latter have been more persistent than the univalves, whether gasteropoda or cephalopoda.
He has not himself given us an analysis of the results of his treatise, but the following tables have been drawn up for me by Mr. Wood's careful examination of 442 species of mollusca has led. Brachiopoda : 6. Lamellibranchia : 206. Gasteropoda : 230. Norwich Crag : 81. Red Crag : 225. Coralline Crag : 327. Norwich Crag : 69 : 12 : 85%. Red Crag : 130 : 95 : 57%. Coralline Crag : 168 : 159 : 51%.
The Caradoc sandstone was originally so named by Sir R.I. Murchison from the mountain called Caer Caradoc, in Shropshire; it consists of shelly sandstones of great thickness, and sometimes containing much calcareous matter. Nothing is more remarkable in these beds, and in the Silurian strata generally of all countries, than the preponderance of brachiopoda over other forms of mollusca.
The brachiopoda of the Llandeilo flags, which number 47 species, are in the main the same as those of the Caradoc Sandstone, but the other mollusca are in great part of different species. Orthoceras duplex, Wahlenberg. Russia and Sweden. Asaphus tyrannus, Murchison. Ogygia Buchii, Burm. Syn. Asaphus Buchii, Brongn.
To reduce the table within limits, I have grouped together all the lower forms of life in the animal table, viz., the sponges, corals, encrinites, and molluscs. It is sufficient to say that these appear in all the rocks except the very oldest the Caelenterata beginning, and the Molluscoids exhibiting an early order in brachiopoda, which seems to be dying out.
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