GEOGNOSY AND GEOLOGY. 181 
sufficient size (sometimes an inch in diameter) to permit this fracture 
(fig. 10). A nummulite consists of an aggregation of spirally wound 
chambers, separated from each other by obliquely disposed, almost vertical 
partitions. #ig. 11 is a cross-section. 
Certain Echini are very characteristic of single beds of the cretaceous. 
In the white chalk Galerites is distinguished in this respect : G. albo-galerus 
(fig. 12). This genus belongs to the Clypeasters, known by their nearly 
conical shape, and by the occurrence of the ten-cornered mouth in the 
middle of the base, with the anus in the posterior border. The five 
ambulacral zones run directly from the vertex to the mouth. The plates 
are well defined, and possess feeble tubercles, upon which stand slightly 
developed spines. 
The Hippurites, or Rudistes, come near to the Brachiopoda, and are highly 
inequivalved bivalves. The lower shell, attached either below or at the side, 
is much the larger, and is closed by a smaller operculoid valve, without 
hinge or ligament. The figure is very irregular, as they formed beds like 
oysters, and of course were obliged to accommodate themselves to the 
spaces left vacant by the contiguous individuals. The shells of the 
Hippurites are very thick, yet, nevertheless, so often destroyed that only 
stony casts, in the form of two cones, placed base to base, remained behind, 
and were known at an earlier period under the name of Birostrites or 
Jodamia. Considerable uncertainty was for a long time felt as to the true 
place of these animals in the scale, and the divisions in the lower part of the 
shell gave rise to the erroneous idea that they belonged among the 
cephalopods. This supposition is now, however, completely refuted, and 
the study of the closely allied Crania enables us to place it with a 
considerable degree of certainty among or near the brachiopods. 
The Rudistes separate into several genera, particularly into the true 
Hippurites and the Spherulites. The former have a very long conical 
lower shell, with acute base and several longitudinal furrows. The much 
smaller upper shell is very flat and operculoid, and this contributes to the 
formation of a simple conical stone nucleus: H. organisans ( pl. 39, fig. 14) 
and H. bioculata (fig. 15). 
The lower shell of Spherulites is smaller and the upper larger than in 
the preceding. It therefore appears as two unequal cones, placed base to 
base, with ridged and often foliated surface: S. ventricosa [ Radiolites 
turbinata] { fig. 16). 
Among the true Brachiopoda may be mentioned Crania, whose small. 
almost circular shells, are attached by their lower portion. The lower 
shell is flat, and has a process, on whose sides are two deep muscular 
impressions, which also exist on the upper free comical valve; the process 
and valve impressions are sometimes wanting. The border of the shell is 
provided with warty elevations. Fig. 17 represents the inner part of the 
upper shell; fig. 18, the same from below. 
Trigonia aliformis and T. scabra (fig. 20), and Cardium productum 
( fig. 21), represent their respective genera. 
The Ostracee are quite different in form from that which we found te 
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