. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 240 HISTORICAL Fig. 172.—Beloteii- tkis subcostata. Jur- assic (Lias). and two lateral wings, somewhat like a feather in general shape. When, on the other hand, the internal skeleton is calcareous, then it is very easily preserved in a fossil condition ; and the abundance of remains of this nature in the Secondary rocks, combined with their apparent total absence in Palaeozoic strata, is a strong pre- sumption in favour of the view that
. The ancient life-history of the earth; a comprehensive outline of the principles and leading facts of palæontological science. Paleontology. 240 HISTORICAL Fig. 172.—Beloteii- tkis subcostata. Jur- assic (Lias). and two lateral wings, somewhat like a feather in general shape. When, on the other hand, the internal skeleton is calcareous, then it is very easily preserved in a fossil condition ; and the abundance of remains of this nature in the Secondary rocks, combined with their apparent total absence in Palaeozoic strata, is a strong pre- sumption in favour of the view that the order of the Cuttle-fishes did not come into exis- tence till the commencement of the Meso- zoic period. The great majority of the skele- tons of this kind which are found in the Jur- assic rocks belong to the great extinct family of the " Belemnites" (Bde?nnitidce), which, so far as known, is entirely confined to rocks of Secondary age. From its pointed, gener- ally cylindro - conical form, the skeleton of the Belemnite is popularly known as a "thun- derbolt " (fig. 173, C). In its perfect condition —in which it is, however, rarely obtainable— the skeleton consists of a chambered conical shell (the "phragmacone'"), the partitions between the chambers of which are pierced by a marginal tube or " ; This conical shell—curiously similar in its structure to the external shell of the Nautilus—is extended forwards into a horny "pen," and is sunk in a corresponding conical pit (fig. 173, B), excavated in the substance of a nearly cylindrical fibrous body or "guard,'' which projects backwards for a longer or shorter distance, and is the part most usually found in a fossil condition. Many different kinds oi Belemnites are known, and their guards literally swarm in many parts of the Jurassic series, whilst some specimens attain very considerable dimensions. Not only is the internal skeleton known, but specime
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Keywords: ., bookaut, bookcentury1800, bookdecade1870, booksubjectpaleontology
