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Updated: June 17, 2025
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.
A case less open to the objection of negative evidence, however, is that afforded by the Tetrabranchiate Cephalopoda, the forms of the shells and of the septal sutures exhibiting a certain increase of complexity in the newer genera.
These zoological results are curious and unexpected, since it was imagined that we might look in vain for the carnivorous trachelipods in rocks of such high antiquity as the Great Oolite, and it was a received doctrine that they did not begin to appear in considerable numbers till the Eocene period, when those two great families of cephalopoda, the ammonites and belemnites, and a great number of other representatives of the same class of chambered shells, had become extinct.
To this day, the last representative of the Cephalopoda with partitioned shells, the Nautilus of the Southern Seas, remains faithful to the ancient design; it has not improved upon its distant predecessors.
A case less open to the objection of negative evidence, however, is that afforded by the Tetrabranchiate Cephalopoda, the forms of the shells and of the septal sutures exhibiting a certain increase of complexity in the newer genera.
Indeed, some of our rock-deposits consist chiefly of the fragments of their remains. The Mollusks were also represented then, as now, by their three classes, Acephala, Gasteropoda, and Cephalopoda. The Acephala or Bivalves we shall find in great numbers, but of a very different pattern from the Oysters, Clams, and Mussels of recent times.
Plagiostoma Hoperi, Sowerby. Syn. Lima Hoperi. The Cephalopoda are abundant, among which 40 species of Ammonites are now known, 10 being peculiar to this subdivision, and the rest common to the beds immediately above or below. Ancyloceras spinigerum, d'Orb. Syn. Hamites spiniger, Sowerby. Near Folkestone.
A case less open to the objection of negative evidence, however, is that afforded by the Tetrabranchiate Cephalopoda, the forms of the shells and of the septal sutures exhibiting a certain increase of complexity in the newer genera.
It is said truly that the Ammonite, Orthoceras, and Nautilus of these ancient rocks were of the tetrabranchiate division, and none of them so highly organised as the Belemnite and other dibranchiate cephalopods which afterwards appeared, and some of which now flourish in our seas. Therefore, we may infer that the simplest forms of the Cephalopoda took precedence of the more complex in time.
The object of this kind of inquiry is to ascertain what are the uses of organs or structures, what they are for, as we say in colloquial language, to discover what are their functions and how these functions are useful or necessary to the life of the animals or plants to which they belong. For example, some Cuttle-fishes or Cephalopoda have eight arms or tentacles and others ten.
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