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many organic remains, or such as can be made available for the 
present purpose. 
For many reasons it is desirable to restrict our investigation 
as regards the succession of life in the above-mentioned de- 
posits to a single group of organisms, and for this purpose none 
offers such facilities as that of the Brachiopoda. It will not 
be possible, indeed, to study even these in an exhaustive manner, 
and only the more striking facts brought to light by a consi- 
deration of their occurrence in these formations can here be 
discussed. 
In the first place, it is most remarkable to observe how in 
the larger and most abundantly represented genera closely 
allied forms succeed each other as we proceed from the base of 
the series towards the summit. Commencing with a single 
type-form in one of the lower groups, we find the same form 
under a somewhat different guise appearing in one or more of 
the higher groups, and sometimes represented therein by several 
allied species. I shall give some of the more conspicuous 
examples of this, drawn from a study of the genera Orthis, 
Strophomena, and Spirifera. 
If we commence, for example, with Orthis elegantula, Dalm., 
in the Clinton group, we have a well-known type nearly allied to 
certainLowcr Silurianforms (such as O.testudinaria, Dalm.), and 
distinguished by its flattened dorsal and convex ventral valve, 
and by the fine radiating dichotomising strice with which the 
surface is ornamented. In the Niagara group the species is 
continued in full force, and little or not at all changed ; but in 
the Lower Helderberg the species has disappeared, and its 
place is taken by the closely allied Orthis plano-convexa, Hall, 
and O. subcarinata, Hall. In the Oriskany Sandstone no re- 
presentative of the type has yet been detected, but in the 
Corniferous group we find Orthis peloris, Hall, and O. lenti- 
cularis. Hall, whilst the Hamilton group has yielded O. soli- 
taria, Hall; all of these being close allies of one another, and 
of O. elegantula. 
Another series may be taken, having as its type Orthis 
hybrida, Sow. This type commences in the Clinton group in 
the person of O. circulus, Hall, and is represented in the suc- 
ceeding formation of the Niagara group by the type-form O. 
hybrida, distinguished by its nearly equally convex valves and 
fine radiating striae. In the Lower Helderberg the type has 
a great development, being represented by O. oblata, Hall, O. 
discus, Hall, O. eminens, Hall, O. perelegans, Hall, O. con- 
cinna. Hall, and O. assimilis. Hall; all of these being closely 
related to O. hybrida and to one another. In the Oriskany 
