This wholesale substitution, adopted by Linnæus, as a ready method of avoiding a tedious revision of all the headings, when he absorbed in the more comprehensive groups of his 'Systema' the members of manuscript genera he had determined to reject, involved a serious amount of confusion; for, oftentimes, the species of the two works, although designated by the same appellations, were totally distinct; and the combination of the diagnosis of the one with the details of the other displayed an array of features not known to be associated in any object in nature.
The generic arrangement exhibited in the manuscript differs essentially from that which appeared in the final edition of his 'Systema Naturæ.' As a whole, it is decidedly inferior, yet it segregates certain natural groups, such as Lyra and Cassida, the value of which have been acknowledged by all modern naturalists. The following list and sequence of the genera comprised in it, cannot, indeed, be regarded as an entire system, for certain groups, viz., Chiton, Lepas, Teredo, Sabella, and the typical forms of Mya, Mactra, and Anomia, were not at that period represented in the Museum; but it is not devoid of interest, since it manifests a transitional stage in the progressive advance to that matured scheme which was finally elaborated in the pages of his revised 'Systema.'
- Dentalium.
- Patella.
- Nerita.
- Helix.
- Turbo.
- Trochus.
- Turricula.
- Buccinum.
- Lyra.
- Morion.
- Conus.
- Voluta.
- Strombus (not that of the 'Systema').
- Harpago (=Strombus).
- Murex.
- Cassida.
- Cypræa.
- Bulla.
- Haliotis.
- Nautilus.
- Cymbium (=Argonauta).
- Spondylus.
- Ostrea.
- Pecten.
- Arca.
- Pinna.
- Mytilus.
- Solen.
- Tellina.
- Chama (not that of the 'Systema').
- Cunnus (=Venus).
- Pholas (not that of the 'Systema').
- Trunculus (=Donax).
- Bucardium (=Cardium).
Besides the four genera (Chiton, Lepas, Teredo, Sabella) that were excluded from this catalogue, either from the absence of specimens, or from mistrust of their being veritable Testacea, six of the remaining 32, namely, Pholas, Mya, Mactra, Chama, Anomia, and Serpula, were likewise omitted, not being yet eliminated from Solen, Bucardium, Spondylus, Ostrea, and Dentalium. To counterbalance these, we find no less than eight subsequently abandoned groupings:
Turricula (an undefined amalgam of the long-spired species of Buccinum, Murex, and Strombus).
Lyra (the Harpa and Purpura of the Lamarckian school).
Morion (an unnatural compound of Eburna, Auricula proper, Pythia, &c.).
Strombus (a combination of the immature members of the received genus with Pyrula, Fasciolaria, and other allied forms).
Cassida (nearly the modern Cassis).
Pecten (equal to Lima and Pecten).