ORTHIDA. 205 
Famity—ORTHID A. 
Genus—OrtHIs, Dalman, 1827. 
See “General Introduction,” p. 101. 
Orthis is one of the most important genera that occur in the Silurian rocks of this 
and other countries; it will consequently be necessary to devote to it very careful study. 
It varies considerably in shape, as do the species of every great genus; and it is pro- 
bable that, when completely understood or worked out, it may be necessary to sub- 
divide it into several minor groups, distinguished by certain internal and external 
characters and features. It is also one of the earliest genera with which we are at 
present acquainted, for one species has been found in the Lower Lingula-flags, or 
Menevian group ; and the genus has continued to prevail throughout the entire series of 
Palzeozoic rocks. 
Before describing the many species that occur in Great Britain, it may be as well 
to briefly notice a few of the most prominent features in the interior of each valve. 
These details differ, however, somewhat in certain forms, but the same general character 
appears to be pretty constant in by far the larger number. 
Orthis elegantula, Dalman. 
Fie. 1.—Interior of the dorsal valve :—E. Cardinal process. C. Brachial processes. S. Dental sockets, usually sur- 
rounded by a raised ridge, which we will designate as the socket-walls. 4’. Posterior adductor or occlusor 
muscular impression; of these there are two, separated by a ridge of greater or lesser width; under these 
are another pair of muscular scars, 4°, similarly separated, which have been referred to the anterior 
adductor or occlusor muscle. The form of these four scars varies in some species. 
Fre. 2.—Interior of the ventral valye:—M. Area. T. Teeth, at times supported by strong plates, termed dental 
plates. 4. Adductor or occlusor muscular impression, usually situated on either side of a slightly elevated 
ridge. D. Divaricator muscular impressions. O. Ventral adjustor muscular impression. These three kinds 
of scars are usually enclosed in a saucer-shaped depression or cavity, more or less distinctly surrounded 
by a prominent ridge, which is sometimes formed by a prolongation of the dental plates. 
