BRACHIOPODA. 171 



There are certain coarse-ribbed variations of the typical A. reticularis occur- 

 ring in the Upper Silurian faunas of Great Britain, Sweden and Bohemia, 

 which appear to be unrepresented in North America. These have sometimes 

 received the designations of var. aspera or Murchisoniana, but writers who have 

 dealt with them agree that they are connected by insensible gradations with 

 the typical form of the species. These seem to us to be simply instances of 

 individual variation due to a deficiency in the usual bifurcation of the plica- 

 tions, and leading to no such distinct specific expression as that borne by 

 Atrypa rugosa, of the Niagara group. Yet to fully apprehend the fundamental 

 relations of the species Atrypa reticularis to the species A. rugosa, it is necessary 

 to have recourse to extremely young conditions of the species. Figure 1, on 

 Plate LIV, represents the earliest growth-stage of A. reticularis observed, the 

 shell having a length of 2.2') mm. This is still a secondary condition of 

 growth, as shown by the two concentric varices and the well-developed plica- 

 tions, but the simplicity of the latter and their relatively great size is a char- 

 acter continued to much later growth (see, for example figs. 21, 22, on plate xiv, 

 of Davidson's Silurian Brachiopoda). When duplication begins, it is carried on 

 with great rapidity in the development of the typical form. It is thus evident 

 that coarse and sparsely duplicated ribs accompanying normal adult size imply 

 a continuance of immature conditions, or an early deficiency of development ; 

 and this genetic modification is the more forcibly expressed when the size of 

 the adult is small, as in A. rugosa.* 



However strong the presumptive evidence may be, that the typical or finely 

 plicated Atrypa reticularis, and the coarse-ribbed forms known as A. rugosa, 

 A. aspera, etc., have originated from a common source, we can not yet indicate 

 the form to which they are both united by an uninterrupted transition. At all 

 events, from the opening of the Upper Silurian to the close of the Devonian 

 period, the two types of external structure have led an independent existence. 

 Though in American faunas, the line of descent of A. reticularis is interrupted 



• The simple exterior of this fossil suggests its relation to the still smallei-, coai'sely plicated shells which 

 have been placed under the genus Atrtpina {A. disparilis, Niagara group ; A. imbricata, Lower Helder- 

 berg group). In the structure of its brachidiuni and the direction of the spiral cones, A. rugosa is a true 

 Atbtpa, although its loop is continuous. 



