GENUS RHYNCHONELLA. 333 



we should hold such views with reservation. In the present instance, I 

 must be allowed to doubt whether R. loxia will be found to possess the - 

 characters of Lower Silurian and of existing Rhynchonell^ ; nor do I 

 consider the characters of the existing species as congeneric with those 

 of the older Silurian or Devonian formations. The necessity felt for some • 

 other designation to apply to some of these forms has induced the names 

 Hypothyris, Hemithyris, Cydothyris, etc. ; but these do not appear to have 

 been founded on reliable structural, characters. That some subdivision 

 will become necessary, and will be adopted, I have no doubt; but such a 

 desirable end can only be consummated after the study of the interiors 

 of numerous specimens, with large collections for comparison.* 



Although liaviiig adopted the name Ehynchonella for our Devonian species, I 

 have lately observed characters which api)car to me to separate them so widely 

 from the recent species of the genus, that I am compelled to substitute some other 

 designation. 



Among genera of the same family, we must necessarily' make distinctions upon 

 the modifications of certain parts which they may all possess in common ; and 

 the importance of these modifications of interior parts or appendages should not 

 be overlooked. Not only do the recent and fossil forms referred to Ehynchonella 

 possess great similarity in the ventral valve, teeth and dental plates, but they are 

 not very dissimilar from the same parts in genera of the SpiRiFERiDiE. It is in the 

 dorsal valve and its appendages that we find characters the most important and relia- 

 ble for generic distinction ; and it is only necessary to follow these in the genera 

 of the Terebratulid.e and Smriferid.t:, to recall the most curious and interesting 

 modification of the parts which this valve sup^iorts. I am therefore inclined to 

 regard these modifications of generic importance. 



In many of the fossil species referred to RirrxcHOXELLA, one of the most con- 

 spicuous features in the dorsal valve is the strong septum, which becomes broader 

 and often shows indications of division at the apex, or at least evidence of a 

 small and shallow V-shaped pit. 



• The reliance upon external forms for the determination of generic affinities or differences, although 

 a compulsory condition in many instances, is far from satisfactory. As an example of this kind, I may 

 remark that having recently occasion to compare Ttrebratula sappho, Barbandb, with RhynchoneUa 

 snppho. Hall, the former presented so much of a rhynchonelloid aspect as to suggest the propriety of 

 "regarding the latter as a synonym. A careful examination of the Bohemian species, from cutting down 

 the solid fossil, has revelled the fact that it is furnished with calcareous spires arranged as in Atrypa, 

 the spires directed into the cavity of the dorsal valve, and the crura connected by a loop in a similar 

 manner. 



