PLAGIOSTOMA. MONOMYARIA. 81 
marine bodies. Their external surface is invariably rough, and clothed with 
spines or foliations. The upper valve is always more intense in its coloration 
than the lower one. 
The teeth of the lower valve are so much arcuated, and inserted so peculi- 
arly into their receiving sockets in the opposite valve, that it is impossible to 
separate the valves without either breaking them, or the margins of the pits. 
Although the ligament appears to be entirely internal, it is in fact also partly 
external, which portion is but very slender, and attaches the valves to each 
other along their linear margins. 
Fossil Spondyli are rare, and occur in tle Blue Marls, south of France, and 
in the Supercretaceous rocks of Bordeaux and Dax. 
Genus XXV.— PLICATULA. — Zamarck. 
Generic Character. — Shell irregular, inequivalve, and 
destitute of ears, attenuated at the base, rounded and 
plaited at the upper margin; umbones unequal and entire ; 
hinge with two strong, generally perpendicularly grooved 
teeth in each valve, with their points recurved, and a cen- 
tral cavity or pit for the reception of the ligament, which is 
internal; under valve generally more convex than the upper 
one; muscular impressions strong, orbicular, and situate 
near the centre of the valves. 
Plicatula spinosa. Plate VII. fig. 2, P. Pectinoides, 
Plate VIII. fig. 14. 
This genus has some affinity to that of Spondylus, in which it was, for the 
most part, included, until separated by Lamarck, but it will at once be dis- 
tinguished by its being destitute of ears, and in never having the recurved 
teeth of that genus, nor are the valves separated by an external area, The 
teeth of this genus are so fitted into their receiving pits that they cannot be 
separated without injuring the shells; nor will they even admit of the valves 
being opened wide. The shells of this genus inhabit the seas of tropical and 
intertropical regions. 
Some Fossil species occur in the Lias, the Blue Marls of France, in the 
Sussex Chalk, and in the Oolitic group of rocks. 
Genus XX VI.— PLAGIOSTOMA. — Liluyd. 
Generic Character. — Shell inequilateral, sub-equivalve, 
oblique and provided with small ears, mostly higher than 
long ; generally covered with grooves or strie diverging 
from the umbones, and passing to the ventral margin ; base 
of the hinge transverse, straight, and destitute of teeth; um- 
bones remote ; depression for the ligament, either straight, 
or slightly angular. 
Plagtostoma punctata. Plate VII. fig. 5. 
The Plagiostomata, seem to have been attached by a byssus in their living 
condition, there being a passage through the anterior part of the shell, which 
in most of the species is rather open. ; 
The shells of this genus are known only in a fossil state, and most important 
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