. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palaeontological science. Paleontology. 112 HISTORICAL PALAEONTOLOGY. which it is organically connected by muscular attachments. The head is furnished with long muscular processes or " arms/'. Fig. 54.—Different views oi Maclurea cren-ulata, Quebec Group, Newfoundland. (After Billings.) and can be protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or &
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palaeontological science. Paleontology. 112 HISTORICAL PALAEONTOLOGY. which it is organically connected by muscular attachments. The head is furnished with long muscular processes or " arms/'. Fig. 54.—Different views oi Maclurea cren-ulata, Quebec Group, Newfoundland. (After Billings.) and can be protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or " gills ;" hence all these forms are grouped together under the name of the " Tetrabranchiate " Cephalo- pods (Gr. tetra, four; bragchia, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered ; their " arms " are furnished with powerful organs of adhesion in the form of suckers ; and they possess only a single pair of gills. For this last reason they are termed the " Dibranchiate " Cephalopods (Gr. dis, twice; h-agchia^ gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great num- ber of forms, sometimes of great size. The principal Lower Silurian genus is the well-known and widely-distributed Ortho- ceras (fig. 55). The shell in this genus agrees with that of the existing Pearly Nautilus^ in consisting of numerous chambers separated by shelly partitions (or septa), the latter being per- forated by a tube which runs the whole length of the shell after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are sup- posed to act as a kind of float, enabling the cr
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Keywords: ., bookcentur, bookdecade1870, booksubjectpaleontology, bookyear1876
