Family 20.—GASTROCHÆNIDÆ. (5 Genera, 40 Species.)
Distribution.—Temperate and warm seas. Aspergillum ranges from the Red Sea to New Zealand. There are 35 fossil species, ranging back to the Lower Oolite.
Family 21.—PHOLADIDÆ (4 Genera, 81 Species.)
Distribution.—These burrowing molluscs inhabit all Temperate and warm seas from Norway to New Zealand. There are about 50 fossil species, ranging back to the epoch of the Lias.
General Remarks on the Distribution of the Marine Mollusca.
The marine Mollusca are remarkable for their usually wide distribution. About 48 of the families are cosmopolitan, ranging over both hemispheres, and in cold as well as warm seas. About 15 are restricted to the warmer seas of the globe; but several of these extend from Norway to New Zealand, a distribution which may be called universal, and only 2 or 3 are absolutely confined to Tropical seas. Two small families only, are confined to the Pacific and Indian Oceans. Marine fishes, on the other hand, have a much less cosmopolitan character, no less than 30 families having a limited distribution, while 50 are universal. Some of these 30 families are confined to the Northern seas, some to the Atlantic and Mediterranean, and a considerable number to the Indian Ocean and Western Pacific. Many of these families, it is true, are much smaller than those of the Mollusca, which seem to possess very few of those small isolated families of two or three species only, which abound in all the Vertebrate classes. These differences are no doubt connected with the higher organisation of fishes, which renders them more susceptible to changed conditions of life; and this is indicated by the much less antiquity of existing families of fishes, the greater part of which do not date back beyond the Cretaceous epoch, and many of them only to the Eocene. In striking contrast we have the vast antiquity of most of the families of Mollusca, as shown in the following table of their range taken from Mr. Woodward's work, but re-arranged, and somewhat modified.
| Range of Families of Mollusca in Time; arranged in their order of appearance and disappearance. | 1. | 2. | 3. | 4. | 5. | 6. | 7. | 8. | 9. | 10. | 11. | 12. | 13. | 14. | |
| Productidæ | — | — | — | — | — | ||||||||||
| Orthoceratidæ | — | — | — | — | — | — | |||||||||
| Spiriferidæ, Orthidæ | — | — | — | — | — | — | — | ||||||||
| Atlantidæ, Hyaleidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Pyramidellidæ, Turbinidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Ianthidæ, Chitonidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Lingulidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Aviculidæ, Mytilidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Arcadæ, Trigoniadæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Cyprinidæ, Anatinidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Nautilidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | ||
| Rhynchonellidæ, Craniadæ, Discinidæ |  | — | — | — | — | — | — | — | — | — | — | — | — | — | |
| Cardiadæ, Lucinidæ | — | — | — | — | — | — | — | — | — | — | — | — | — | ||
| Ammonitidæ | — | — | — | — | — | — | — | — | |||||||
| Naticidæ, Calyptræidæ | — | — | — | — | — | — | — | — | — | — | — | — | |||
| Dentalidæ, Terebratulidæ | — | — | — | — | — | — | — | — | — | — | — | — | |||
| Helicidæ | — | — | — | — | — | ||||||||||
| Fissurellidæ, Tornatellidæ | — | — | — | — | — | — | — | — | — | — | — | ||||
| Pectinidæ, Solenidæ | — | — | — | — | — | — | — | — | — | — | — | ||||
| Cerithiadæ, Littorinidæ, Astartidæ | | — | — | — | — | — | — | — | — | — | |||||
| Belemnitidæ | — | — | — | — | |||||||||||
| Teuthidæ, Sepiadæ | — | — | — | — | — | — | — | — | |||||||
| Neritidæ, Patellidæ, Bullidæ | | — | — | — | — | — | — | — | — | ||||||
| Gastrochænidæ, Pholadidæ | — | — | — | — | — | — | — | — | |||||||
| Limnæidæ, Melaniadæ | — | — | — | — | — | — | — | ||||||||
| Chamidæ, Myadæ | — | — | — | — | — | — | — | ||||||||
| Cycladidæ, Veneridæ, Tellinidæ | | — | — | — | — | — | — | — | |||||||
| Hippuritidæ | — | — | |||||||||||||
| Unionidæ | — | — | — | — | — | — | |||||||||
| Strombidæ, Buccinidæ | — | — | — | — | — | — | |||||||||
| Conidæ, Volutidæ | — | — | — | — | — | — | |||||||||
| Auriculidæ, Cyclostomidæ | — | — | — | — | — | — | |||||||||
| Mactridæ | — | — | — | — | — | — | |||||||||
| Limacidæ | — | — | — | — | |||||||||||
| Argonautidæ | — | — | |||||||||||||
| Tridacnidæ | — | — | |||||||||||||
Nor is this enormous antiquity confined to family types alone. Many genera are equally ancient. The genus Lingula has existed from the earliest Palæozoic times down to the present day; while Terebratula, Rhynchonella, Discina, Nautilus, Natica, Pleurotomaria, Patella, Dentalium, Mytilus and many other living forms, range back to the Palæozoic epoch. That groups of such immense antiquity, and having power to resist such vast changes of external conditions as they must have been subject to, should now be widely distributed, is no more than might reasonably be expected. It is only in the case of sub-genera and species, that we can expect the influence of recent geological or climatal changes to be manifest; and it must be left to special students to work out the details of their distribution, with reference to the general principles found to obtain among the more highly organised animals.
CHAPTER XXIII.
SUMMARY OF THE DISTRIBUTION, AND LINES OF MIGRATION, OF THE SEVERAL CLASSES OF ANIMALS.