152 GEOGNOSY AND GEOLOGY. 
muscular arms, serving the purposes of prehension, for which they are well 
calculated by reason of the numerous sucking disks on their inner face: 
these suckers are sometimes still further armed by formidable hooks. In 
the intestines there exists a sac filled with an inky fluid, and which in some 
cases has with its contents been preserved in a fossil state. This fossil 
sepia or India ink has even been used to delineate the fossil remains with 
which it was associated. The Sepie of the present day rarely have external 
shells; the animal, however, incloses a solid shelly axis, known in the arts 
as cuttle fish bone (ossa sepia). The analogous parts of somewhat similar 
fossil forms are frequently met with in fossiliferous strata. Only two of the 
families into which the Cephalopoda are divided, occur in the transition 
series: the Nautilide and Ammonitide. These have chambered shells, 
the last chamber of which was inhabited by the animal. The young 
individual formed only one cell, others being successively added, and the 
last built being the only one occupied. A membranous tube called the 
sipho passed through to the last chamber, and was connected as to its 
function with the rising or sinking of the animal in the water. The position 
of the sipho, whether passing through the middle of the partitions, along 
the ventral or along the dorsal, characterizes the subdivisions of the 
chambered cephalopods. 
The aggregation of chambers is sometimes in a straight line, widening 
above, as in Orthoceras; of this genus, O. attenuatum (fig. 45) and Q. 
annulatum (pl. 42, fig. 63) are of most frequent occurrence in the transition. 
The partitions are slightly concave, the sipho passing through the middle, 
Phragmoceras has a structure somewhat similar, except in being slightly 
curved below into a horn shape: P. ventricosum (pl. 37, fig. 46). They 
only occur in the transition state. Lituites is rolled up, yet without any 
contact of the windings: L. giganteus (fig. 47). 
The Ammonitide are distinguished from the Nautilide by the more or 
less undulating or zigzag character of the partitions, whose extremities are 
very distinct externally. The sipho les nearer the dorsal side. In the 
Nautilide the partitions are simply curved with the sipho in the middle 
or nearer the ventral side. Here belongs Goniatiles, as G. hoeninghausi 
(pl. 37, figs. 48 and 49), and G. costulatus (fig. 50 and 51). 
The class Crustacea is represented in the oldest fossiliferous strata by a 
very remarkable form, that of the Trilobites. No adult crustacean of the 
present day is at all similar to the Trilobites: a very striking resemblance 
is, however, found in the embryo of some recent species. The body of the 
animal was divided into three principal portions, a head, a thorax, and tail ; 
these were covered by a thin granular or spinous shell, which has rarely 
been preserved, casts only of these portions being generally exhibited. The 
head is occupied by a large shield, with a large eye on each side: these may 
sometimes be recognised as compound by means of numerous facets. The 
thorax consists of a central longitudinal rilge, with a furrow on each side, 
and is divided transversely into jointed rings, the number varying from 5 to 
20. The caudal shield is divided into similar rings, the central elevation of 
the thorax being lost in it. Some genera were able to roll themselves up 
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