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 imbricata, Sowerby, which is a similar but highly im- 

 bricated shell, whose resemblance to Atrypa rugosa of the Niagara group 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,'RTi\\, 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 iu 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 thera. 



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 



