BRACHIOPODA. 
167 
retaining only those which conform to the well-known A, reticularis, primarily 
in the structure of the brachidium, and secondarily, in the expression of the 
exterior. Such forms are comparatively few in number, and most authors 
have been disposed to regard them as representing unessential variations from 
the specific type of A. reticularis. There is, however, a multitude of designations 
which have been applied to contemporaneous variations or consecutive muta¬ 
tions of this specific type, some of them unnecessary, but many very useful both 
to the geologist and the systematist.* 
Atrypa reticularis is a shell characterized by its fine plications, which duplicate 
rapidly at or between the concentric growth-lines. This duplication or bifur¬ 
cation of the plications occurs at irregular intervals in the growth of the shell. 
It is a secondary condition of growth and if it manifests itself at an early stage, 
a finer plication results than when its appearance is delayed until later growth. 
This variability in appearance and rapidity of recurrence produces individual 
differences of expression in the plication of the shell, which, however, lead to 
no varietal modifications. 
The concentric growth-lines are bases of free squamse or lamellae, which under 
favorable conditions may be retained, but are usually abraded, so that the com¬ 
mon expression of the exterior is that of an entire absence of such growths. 
This is the condition where the valves have been replaced by silica (a very 
common mode of retention), or in specimens which have been gathered from 
compact limestone. Under better preservation, as in soft shales or shaly lime- 
=*= The time-values of oscillations of, or from the specific type, manifest themselves so clearly in this g-enus, 
that it is here necessary to express such variations with caution and ])recision. Barrande introduced a dis¬ 
tinction between pi'imary and secondary modifications of a sp)ecific type, by proposing to restrict the tei'm 
varitty to the former, that is, “to forms which jiossess the principal characters of an admitted sjiecies, but 
which differ fi-om it in one or more important modifications, manifesting themselves in a considerable number 
of individuals ; ” and to designate as variants, secondary modifications of foian and surface ornamentation. 
It would be granted by most investigators that modifications of a specific type moi-e essential than changes 
in form and surface charactei’S, would be a sufficient basis for a complete separation from such species; 
hence this distinction between varitiy and variant, holding the latter subordinate to the former, becomes 
largely arbitrary ; and it is only in rare instances that any practical use can be made of it. Variant becomes 
a useful term applied to the different phases of expression within the limit of the specific type, but in this 
meaning it is neither subordinate to the tei m variety, nor does it necessarily indicate an inceptive condition 
in the departure of a vai'iety from the specific type. The time-value of variations from the species has been 
expressed by Waagen, who proposed to restrict the tei-m variety to oscillations of the type contemporaneous 
with the type itself, and applied the term mutation to variations appearing after the extinction of the type. 
