240 



HISTORICAL PALEONTOLOGY. 



Fig. 



this subcostata. Ju 

 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" (BdemnitidcE), 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. 1 73, 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 " siphuncle." 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 of 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 specimens of 

 Belemnites and the nearly allied Belemnoteuthis have been found 

 in some of the fine-grained sediments of the Jurassic formation, 

 from which much has been learnt even as to the anatomy of 

 the soft parts of the animal. Thus we know that the Belem- 

 nites were in many respects comparable with the existing 

 Calamaries or Squids, the body being furnished with lateral 

 fins, and the head carrying a circle of ten " arms," two of 

 which were longer than the others (fig. 173, A). The suckers 

 on the arms were provided, further, with horny hooks ; there 

 was a large ink-sac ; and the mouth was armed with horny 

 mandibles resembling in shape the beak of a parrot. 



Coming next to the Vertebrates, we find that the Jurassic 



