644 



SHELLS OF THE LOWER SILURIAN ROCKS. 



" The second genus, Atrypa (from a privative, an&rpwra), is divided from Spirifer, and includes 

 those species which have a short hinge line without a large area, and are either destitute of a fora- 

 men or possess only a small triangular one. They are rounded shells, and are not furrowed like 

 the typical species of Spirifer ; the internal spiral arms are preserved in some species. Atrypa 

 affinis and similar striated shells, would form another natural group, in which the internal structure, 

 as well as the general form, is different ; for the spiral appendages, if ever they possessed any, do 

 not appear to remain, and there are two short crenated teeth in the hinge ; the species of this di- 

 vision have generally been described as Terebratulce by British authors, but they have acute, not 

 perforated beaks." 



" The genus Orthis (opQog) is another division of Spirifer, no species of which has heretofore 

 been described in England ; it is distinguished from Spirifer by the long narrow hinge and cir- 

 cular flat form of the striated shells." 



ftf Our genus Pentamerus is called Delthyris by the Swedes, but we see no reason for altering the 

 name. If we were well assured of the stability of the genus Delthyris we should remove to it 

 Atrypa galeata, and perhaps one or two other species of Atrypa." 



In consequence of the name of Productus having been changed for that of Leptcena since the 

 earlier chapters of the work were printed, the reader must pardon a slight incongruity which is, 

 however, explained by the double synonym in the description of the shells. For Productus read 

 Leptcena. 



