DESCRIPTIVE REMARKS. XXXV1L 
Fossits oF THE Upper Siturian Rocks. 
No very striking change from those of the Lower Silurians is observ- 
able in the general character of the assemblage of fossils contained in 
the upper division of Silurian strata, until we arrive at its uppermost 
portion, when the first evidence of vertebrata is obtained, by the disco- 
very of the remains of fish in the Ludlow formation ; there is, however, 
a considerable difference in the relative numbers of some of the classes 
of organic remains prevalent in Upper Silurian deposits. 
The following figures of recent typical examples of some of the 
classes of Molluscan shells, prevalent as fossils, are introduced for com- 
parison, and to assist in explaining their various parts and the terms 
used in describing them :— 
Fre. 1.—A Bracutopop SHELL. 
Genus Terebratula.—Sub-genus Waldheimia. W. Australis (recent). 
(Reduced to two-thirds of natural size). 
Figs. A and B copied by permission of Dr. J. E. Gray—Fig. C original. 
A—Interior of Dorsal valve. a. Single adductor impression. 
c. p. Cardinal process. c. m, Cardinai muscles. 
d. Dental sockets. a. ec. Accessory muscles. 
p. Hinge plate. p. ePdicle muscles. 
s. Septum. v. Vent. : 
c. Crura of the loop. r. Attachment of pedicle sheath. 
i. Retlected portion of loop. C.—Exterior of both valves. 
q. Quadruple adductor impression. 
i d. Dorsal valve. 
B.—Interior of Ventral valve. v. Ventral. 
Jf. Foramen. Jf. Foramen. 
d, Deltideum. r. Ribs, or plaits. 
t. Teeth. 2, Lines of growth. 
The Brachiopoda are bivalve Molluscoida, inzequivalve, and all 
having marine habitats. In the living examples the interior cavity of 
the shell is provided with two long oral processes, ciliated on one side, 
termed ‘arms,’ from which the name of the class has been derived. 
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