as distinct species. We have thus ground for believing that 
a considerable expansion may reasonably be given to the philo- 
sophical conception of what constitutes a species. 
There is, however, another aspect of the question to con- 
sider. Side by side with the groups of allied species of Bra- 
chiopods, w r hich we have seen to occur in the Upper Silurian 
and Devonian formations, we must place the comparatively 
rigid, inflexible, and persistent species, such as Strophomena 
rhomboidalis, Wahl., and Atrypa reticularis, Linn., of the same 
formations ; and we have also to consider the new types which 
came into being during the same period, without our being 
able to show that they have descended from pre-existent allied 
forms. As regards the persistent, types, the two first-men- 
tioned are the most important. Strophomena rhomboidalis 
commences in the Lower Silurian, and continues through the 
whole of the Upper Silurian and Devonian, finally dying 
out in the Carboniferous rocks. Though slightly varying in 
shape and size in certain deposits, it remains practically the 
same throughout the whole of this enormous period. Atrypa 
reticularis, Linn., commences in the earlier portion of the Upper 
Silurian, and continues to be represented till close upon the 
termination of the Devonian period. Unlike the preceding, 
it is exceedingly variable in size and in other characters, and at 
least two so-called species, viz., Atrypa impressa, Hall, and A. 
spinosa vel aspera, appear to have been founded upon mere 
varieties of this mutable form. Other species could be men- 
tioned which pass through several sub-groups without apparent 
alteration ; but these two have by far the largest range. Atrypa 
reticularis affords us an instance of a species, which, though 
very variable, and constantly presenting slight modifications of 
different kinds, nevertheless retains its specific stamp for a 
very extended period, and under what must have been very 
variable conditions. On the other hand, we have in Stronho- 
mena rhomboidalis a specific type which endured similarly 
changing conditions, and which survived for an even more pro- 
longed term, but which throughout its entire lease of life never 
exhibited any modifications of even varietal value. 
As regards the appearance of new forms of the Brachiopoda 
during the period which we have had under consideration, it 
will be advisable to look to the genera and sub-genera rather 
than to the species. As each new genus and sub-genus in 
general contains more than one, and often many, species, the 
case is thus rendered quite sufficiently strong for our purpose; 
though it is to be remembered that many new species of the 
old genera are also constantly making their appearance in the 
