98 NEW SILURIC GASTROPODS 



There are but two specimens from which we can describe 

 this species, one of these being the exterior of the shell and the 

 other the mold of the same. Little patches of shell are visible 

 on both specimens, the mold showing a very thin film of organic 

 matter, just sufficient to make the rock black. The most striking 

 feature of the shell, and the one which, together with the 

 asymmetry, forms the chief diagnostic characteristic, is the 

 marked incurving of the growth lines in the posterior portion of 

 the shell, along the steeper slope of the long axis. (PI. I, fig. 4.) 

 For a distance of 6 mm. from the periphery of the shell, the 

 growth lines curve in from both sides, showing that, at the edge 

 of the shell which is invisible, there must be quite a sinus as 

 indicated in fig. 3. Such a sinus represents a simple and 



Fig. 3. Restored peripheral outline of Hercynella patelliformis sp. nov. 

 as suggested by the growth lines in pi. I, fig. 4. x 4/5. 



primitive condition of the much more highly specialized one 

 which is seen in some of the Bohemian forms. It is evident that 

 the ancestral Hercynellas must have had only the slightest radial 

 depression, or perhaps none at all, and this shallow depression 

 would be accompanied by a correspondingly shallow re-entrant 

 in the periphery. If it be assumed, that, as was most likely the 

 case, this radial depression was produced by the appearance of 

 the siphon, then we can easily think of a time when there was 

 no siphon and when the curvature of the shell was regular and 

 without interruption. Moreover, the gradual development of 



