182 GEOGNOSY AND GEOLOGY. 
prevail in the Jura; the family is here. represented by Exogyra, differing 
from the displaced Gryphza by the laterally bent beaks: E. columba 
( fig. 22) and E. sinuata (fig. 23). 
Inoceramus comes rather near to the oysters ; they are inequivalve shells, 
of tolerably triangular form. The opposing beaks are strongly bent, and the 
hinge has a number of indentations: J. concentricus (fig. 24) from the 
oreensand. ) 
Spondylus with inequilateral, spiny shells, has two teeth in the hinge, 
and two strong muscular impressions: S. spinosus (fig. 25) from the white 
chalk. 
Pectunculus, represented in the upper greensand by P. subconcentricus 
(fig. 26), is a genus of shells, equivalve, and closing tight, the curved hinge 
provided with teeth, separated from each other by deep intersections, the 
cardinal area covered with broken lines, and the border provided with fine 
teeth. Closely allied to these is the genus Nucula ; fig. 27 represents a cast 
of LV. pectinata from the lower greensand. 
The species of Opis have two very thick tapering cordiform valves 
provided with very long beaks, and a complicated hinge with a large 
compressed tooth: O. elegans (pl. 39, figs. 29 and 30), from the upper 
greensand. 
The genus Acteonella is a Character tie representative of the Gasteropoda 
in the cretaceous. The individual turns of the shell embrace each other in 
such a manner as that the aperture occupies the entire length. The 
aperture is very narrow, and is contracted still more above by three 
thick ridges. <A. crassa (fig. 31) is peculiar to the chloritic chalk. Avellana 
is much more universally distributed than the preceding, and is characterized 
by its short and rotund shell with crescentic aperture, and three strong teeth 
on the inner margin, the outer border with small long transverse teeth: <A. 
incrassata (fig. 32) from the gault. 
Rostellaria has a subfusiform or turreted spiral shell, with the upper part 
of the aperture ending in a long tube, and the margin dilated into a wing, 
or digitated, the wing sometimes falecate towards the apex. The species 
occur from the upper Jura to the present epoch. A characteristic species for 
the chalk is shown in fig. 33: R. parkinsoni. Pterocera pelagi (fig. 34) 
is peculiar to the white chalk, Natica lyrata (fig. 36) to the chloritic 
chalk. 
The Cephalopoda of the cretaceous system present many points of interest. 
The Nautilide occur with simple windings and simple smooth partitions. 
The Ammonitida, previously so remarkable for the fringed and varied 
attachment of the partitions to the main shell, assume a new character in 
approximating to the earlier simplicity of the preceding family. At the 
first appearance of the Nautilidz, it will be remembered that the single 
chambers were combined in various ways, either in a linear direction, as in 
Orthoceratites, or the axis a little curved at the vertex, and then continued 
in a straight line (Phragmoceraiites) or rolled up below (Lituites), and 
sulaceguenily entirely rolled up in a single plane, as in the true Nautilus and 
Goniatites. The opposite condition of things prevails in the ammonite 
612 
