240 HISTORICAL PALASONTOLOGY. 
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 Palzeozoic 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” (Belemnztide), 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- 
Fig. 172.—Belotex- derbolt” (fig. 173, C). In its perfect condition 
this subcostata. Jur- : : us : 
Aeei(laas): —ain 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 
