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Thirty years ago a great London dealer in shells, himself an able naturalist, told me that there was nothing he had so much reason to dread, as tending to depreciate his stock in trade, as the appearance of a good monograph on some large genus of mollusca; for, in proportion as the work was executed in a philosophical spirit, it was sure to injure him, every reputed species pronounced to be a mere variety becoming from that time unsaleable.
This attitude affords as good evidence to the conchologist that those mollusca lived and died on the spot as the upright position of the trees proves to the botanist that there was a forest over the Chalk east of Cromer. Between the stumps of the buried forest, and in the lignite above them, are many well-preserved cones of the Scotch and spruce firs, Pinus sylvestris, and Pinus abies.
The Mollusca form a very large group of Invertebrate animals; they live on land as well as in the water, but the aquatic species are much more numerous than the terrestrial, and the deep-sea dredgings are constantly bringing to light new forms.
He has not himself given us an analysis of the results of his treatise, but the following tables have been drawn up for me by Mr. Wood's careful examination of 442 species of mollusca has led. Brachiopoda : 6. Lamellibranchia : 206. Gasteropoda : 230. Norwich Crag : 81. Red Crag : 225. Coralline Crag : 327. Norwich Crag : 69 : 12 : 85%. Red Crag : 130 : 95 : 57%. Coralline Crag : 168 : 159 : 51%.
In the interval we must suppose repeated oscillations of level, during which land covered with trees, an estuary with its freshwater shells, and the sea with its Mya truncata and other mollusca still retaining their erect position, gained by turns the ascendency.
But there is no similar break or discordance as we proceed downward, and pass from one to another of the several leading members of the Jurassic group, the Upper, Middle, and Lower Oolite, and the Lias, there being often a considerable proportion of the mollusca, sometimes as much as a fourth, common to such divisions as the Upper and Middle Oolite.
The former belies its name by not frequenting water or feeding on fish. It lives constantly in low damp thickets picking up ground insects, centipedes, and small mollusca.
These, through the courtesy of the authorities in charge, are easily accessible to students who wish to prosecute the study of this wonderful branch of the great mollusca family.
The proof of the finer gradation appears to be forthcoming. Des Hayes and Lyell have concluded that many of the middle Tertiary and a large proportion of the later Tertiary mollusca are specifically identical with living species; and this is still the almost universally prevalent view. But Mr.
The nervous system the more comprehensive term for its organic apparatus is variously developed in different classes and species, and also in different individuals, the volume or mass bearing a general relation to the amount of power. In the mollusca and crustacea we see simply a ganglionic cord pervading the extent of the body, and sending out lateral filaments.
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