BRACHIOPODA. 
173 
This shell is characterized by its sharp median fold and sinus, numerous 
fine fasciculate plications and freedom from concentric lamellae. The expres¬ 
sion of the species is thus quite different from that of A. reticularis, but after 
the introduction of the Wenlock fauna the connection between the two is 
indicated by the Atrypa imhricata, Sowerby, which is a similar but highly im¬ 
bricated shell, whose resemblance to Atrypa rugosa of the Niagara gimip at once 
suggests itself. The type of A. marginalis was not highly variable nor, in 
America, long-lived. A small variety is the A. Calvini, Nettelroth, of the 
Niagara formation at Louisville. After the disappearance of the Niagara 
fauna, however, this group does not return, unless the imperfectly known 
A. pseudomarginalis,llQ\\, oi the Upper Helderberg group, be considered a remote 
descendant. 
All the forms considered above are true Atrypas in the structure of the 
brachidium, so far as that feature is known. No successful attempt has been 
made to demonstrate this structure in the Lower Silurian representatives of 
A. marginalis, but should they prove to possess slightly convergent spiral 
cones, directed toward the middle point of the brachial valve, and a simple 
continuous loop, as in later examples of the species, and most of the early 
forms of A. reticularis, we may seek the source of Atrypa in early Silurian 
times. It seems not to have been a derivative of Zygospira or Catazyga, but 
to have developed in a line essentially parallel with those genera and to have 
had its origin in common with them. 
The variations in exterior form are accompanied by some degree of differ¬ 
ence in the structure of the brachial supports. How far this apparent 
difference is due to the stage of development of the individual has yet 
to be determined. The normal form of the spirals in the mature A. reti¬ 
cularis, is that of laterally compressed cones, the first two or three coils 
of the ribbon being extended beyond the rest along their anterior cur¬ 
vature. In A nodostriata the mature form of the spiral is a cone, which 
narrows quite rapidly above its base, is round and slender, tapering to an 
acute apex which is inclined inward to meet that of its companion; while in 
