GEOGNOSY AND GEOLOGY. 173 
alveolus or phragmocone, with a chambered shell and a sipho quite like that 
of Orthoceratites (fig. 61 and fig. 58’). A rarely preserved horny plate 
was connected with the posterior face of the alveolus (figs. 57 and 58, a). 
Belemnites paxillosus (fig. 59, a) occurring in single strata of the lias in 
countless numbers, is readily distinguished by the channelled apex, shown in 
cross-section by fig. 59,b. B. giganteus (fig. 60) is of immense size, 
sometimes over one foot in length: it characterizes the lower oolite. B. 
hastatus (fig. 61) has a hastate cone, and is found in the Oxford marl. 
The above-mentioned view of the morphological character of the 
Belemnites does not, however, explain the import of the chambered shell 
and the sipho. A more careful investigation of allied structures, both 
recent and fossil, renders it very probable that the animal itself lived in 
these chambers like Nautilus and Ammonites, a view which the occurrence 
of the sipho goes very far to substantiate. The shell of the camerated 
portion is very thin and exceedingly fragile, for which reason it is inclosed 
by a thicker, more solid portion, as in Nautilus. This protecting cover is 
the conical body or rostrum so often met with in a petrified state, and, in 
all probability, has been secreted from a mantle. The animal, probably, 
inhabited the upper chamber, as in the Orthoceratites. Ammonites, and 
Nautili ; its mantle was probably analogous to that of the recent nautilus. 
This view also explains how it is that the solid portion is composed of a 
succession of vertical layers, each one corresponding to achamber. When 
the animal formed a new cell, this had to be enveloped by an additional 
external protecting cover, which, of course, embraced the portion already 
existing. The greater the number of chambers in the phragmocone, the 
thicker and longer the solid portion, and the greater the number of layers 
in the latter. 
The strata of Solnhofen, which inclose so large a number of paleonto- 
logical treasures, also exhibit fossil insects, belonging especially to the family 
of the Libellulide (fig. 62). The characteristic features of these insects, as 
the antenne, the masticatory apparatus, the legs, and the nervatures of the 
wings, are not sufficiently well preserved to permit us to compare them very 
closely with the existing members of this family. Many of them are of 
colossal size, even attaining a length of six or more inches. 
The Jura strata reveal a new page of fossil] ichthyological history. We 
here find the heterocercal fishes, or those with an unequally lobed tail, 
receding into the background, and the homocercal coming forward in 
increasing numbers. The essential difference between the two consists in 
the prolongation of the vertebral column along the upper edge of the cauda’ 
fin in the former, while in the latter, the extremity of the body occupies a 
position nearly symmetrical, with respect to the two lobes or halves of the 
tail. 
The teeth of Strophodus, a genus of the Cestraciontes, are depressed, 
truncated on both sides, and elevated in the middle, without a central 
longitudinal fold. Their axis is somewhat twisted, and the surface some- 
times striated or reticulated ; the root is broad and porous. Pl. 38, fig. 63, 
represents a tooth of S. longidens. The genus Pycnodus is the most 
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