A certain number of species, probably of arctic origin, are common with British and also with East American shores, the former being the more numerous. Species as familiar to us as Lacuna divaricata Fabr., Trichotropis borealis Brod., Pholas crispata L., Mya truncata L., M. arenaria L., Mytilus edulis L., and Modiolaria nigra Gray, occur. The more characteristic genera are Chrysodomus, Volutharpa, Buccinum, Tectura, Scurria, Chiton, Cryptochiton (Cr. Stelleri Midd. is by far the largest known of the Chitonidae, 6 inches long), Tellina and Pecten.
(2) The Californian Sub-region extends from St. Jean de Fuca Straits (lat. 48° N.) to Cape St. Lucas, the Gulf of California belonging to the Panamic sub-region. The northern polar current, which washes the shores of this sub-region throughout their whole extent, prolongs the southward range of the more northern genera, and keeps back those more markedly tropical, the latter, however, creeping northward in the warmer waters of the Gulf of California. Some authorities subdivide this immense stretch of coast-line, as characterised by sub-temperate, temperate, and sub-tropical genera, into the Oregonian, Californian, and Lower Californian provinces.
The characteristic genera are—in the north, Argobuccinum, Zizyphinus, Chlorostoma, Tectura, Scurria, Chiton (Katharina, Mopalia, Tonicia), Cryptochiton, Placunanomia, and Mytilimeria; in the centre, Purpura, Monoceros, Amphissa, Norrisia, Platyodon, Tapes, and Macoma; and, towards the south, Olivella, Chorus, Macron, Pseudoliva, Trivia, and Haliotis.
(3) The Panamic Sub-region extends from the head of the Gulf of California to Payta in Peru (lat. 5° S.). It is exceedingly rich in species, about 1500 having been described. The Mollusca are entirely distinct from those of the Indo-Pacific Region, which, although extending from Natal to the Sandwich Islands, are unable to pass the enormous extent of sea which separates the nearest Polynesian island from the American coast.
On the two sides of the isthmus of Panama there occur certain pairs of species, which, while specifically distant, are evidently closely related to one another. Amongst these are, on the Panamic side, Purpura speciosa, Cypraea cervinetta, Cassis abbreviata, Natica Chemnitzii, and Strombus gracilior, corresponding to Purpura deltoidea, Cypraea exanthema, Cassis inflata, Natica maroccana, and Strombus pugilis, on the Caribbean. It is reasonable to conclude that these “analogous species” are descendants of a stock which was common to both seas when the isthmus was open (probably not later than Miocene times), and which have, since the closing of the isthmus, become modified, some species considerably more than others.
Among the characteristic genera (compare p. [3]) are Conus, Pleurotoma, Terebra, Murex, Purpura, Oliva, Northia, Cantharus, Columbella, Anachis, Cypraea, Strombus, Cerithium, Coecum, Crepidula, Crucibulum, Vitrinella; Tellina, Semele, Tellidora; and Arca.
(4) The Peruvian Sub-region extends from Payta in Peru to about the latitude of Conception in S. Chili (37° S.), being checked from further extension southward by the cold Humboldt current, whose force is distinctly felt as far north as Callao. This cold current thus produces the same results as the similar current which impinges on S. Africa, but has even more effect in decisively separating the fauna on the two sides of the great peninsula, scarcely a single species being common to the western and eastern coasts of S. America. The characteristics of the coast-lines themselves contribute to this result. The Chilian coast is rocky, and descends abruptly to a great depth, while that of Patagonia and Argentina is sandy and very shallow to a great distance from land.
The characteristic genera are Cancellaria, Columbella, Monoceros, Concholepas, Trochita, Fissurella, Chiton; Ceronia, Malletia, and Cumingia. Some of the Californian genera, absent or poorly represented in the Panamic Sub-region, reappear in Chili, e.g. Scurria, Tectura, and Chlorostoma.
(5) The Magellanic Sub-region includes the coast-line and adjacent islands (with the Falklands) from Conception in S. Chili to about Port Melo in Eastern Patagonia (lat. 45° S.).
The principal genera (many of which find a habitat on the gigantic Macrocystis which grows on every rock at low water) are Euthria, Voluta (6 species, one, V. magellanica, the largest known) Monoceros, Photinula, Patella, Chiton; Modiolarca, Malletia, and Mulinia. Several genera characteristic of the Boreal and Arctic sub-regions recur, e.g. Trophon, Admete, Margarita, Puncturella, Cyamium, and Astarte.