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In this case, it would be possible to proceed little by little to a solution more and more plausible, and there would be the more chance of a successful issue the more resolutely we assumed the evolutionist hypothesis. Let us place side by side the eye of a vertebrate and that of a mollusc such as the common Pecten. We find the same essential parts in each, composed of analogous elements.
The shells included in these beds have been named for me by M. d'Orbigny: they consist of: Ostrea Patagonica, d'Orbigny, "Voyage" Part. Pal. 2. Ostrea Alvarezii, d'Orbigny, "Voyage" Part. Pal. 3. Pecten Paranensis, d'Orbigny, "Voyage" Part. Pal. 4. Pecten Darwinianus, d'Orbigny, "Voyage" Part. Pal. 5. Venus Munsterii, d'Orbigny, "Voyage" Pal. 6. Arca Bonplandiana, d'Orbigny, "Voyage" Pal. 7.
The remarks given on the several foregoing shells, show that, in M. d'Orbigny's opinion, the Pecten, Ostrea, Turritella, and Hippurite indicate the cretaceous period; and the Gryphaea appears to Professor Forbes to be identical with a species, associated in Southern India with unquestionably cretaceous forms.
The external surface of the crater slopes at a nearly similar inclination, and is formed by slightly convex ribs, like those on the shell of a pecten or scallop, which become broader as they extend from the mouth of the crater to its base.
Its surface was there formed of gravel, cemented together by calcareous matter; and out of it I extracted the following recent shells, which are in a more perfect condition than those from the upper plain: Calyptraea radians. 2. Turritella cingulata. 3. Oliva Peruviana. 4. Murex labiosus, var. 5. Solen Dombeiana. 7. Pecten purpuratus. 8. Venus Chilensis. 9. Amphidesma rugulosum.
Hence there cannot be any doubt, highly curious though the fact be, that these several fossils, namely, the Hippurites, Gryphaea, Ostrea, Pecten, Turritella, Nautilus, two Terebratulae, and Spirifer all belong to the same formation, which would appear to form a passage between the oolitic and cretaceous systems of Europe.
The shells out of the layers of brown limestone, included in the black calcareous shale-rock, which latter, as just stated, replaces the white siliceous sandstone, consist of: Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. Turritella Andii, d'Orbigny, "Voyage" Part Pal. Astarte Darwinii, E. Forbes. Gryphaea Darwinii, E. Forbes. An intermediate form between G. gigantea and G. incurva.
In the North-east Australian province we have species of Donax, Mactra and Corbula, all apparently new, from the shallower localities; Corbula tunicata, Pectunculus tenuicostatus, and another, from 8 to 11 fathoms, off Cumberland Islands; species of Arca, Pectunculus, Avicula, Pecten, Venus, Circe, Cardium, Cardita, and Erycina, mostly new, from 15 to 17 fathoms in a sandy and shelly bottom off Cape Capricorn.
The incumbent till or boulder clay is about 40 feet thick, but it often attains much greater thickness in the same part of Scotland. Pecten islandicus, Moll. Natica clausa, Bred. Trophon clathratum, Linne. Leda truncata. a. Exterior of left valve. b. Tellina calcarea, Chem. Outside of left valve. b.
Now, the origin of molluscs may be a debated question, but, whatever opinion we hold, all are agreed that molluscs and vertebrates separated from their common parent-stem long before the appearance of an eye so complex as that of the Pecten. Whence, then, the structural analogy?
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