Description of Some Cincinnati Fossils. 



153 



have been discovered. We have never seen but two casts of 

 Tellinomya hilli, though we have seen more than two hundred 

 valves of the shells, many of which preserved the denticu- 

 lated hinge-line. This state of facts in regard to Tellinomya, 

 judging from the specimens we have seen and from the illus- 

 trations in the books, prevails throughout the Lower Silurian 

 rocks of North America, and we are not the first, by any 

 means, to call attention to it: for Billings, nearly thirty years 

 ago, in describing T. angcla, said: "The shell is very thick, 

 and rather strongly convex." Others have remarked upon 

 the great thickness of the shells, and nearly every species 

 described is accompanied with an illustration to show the 

 thick denticulated hinge-line. This is what we call the 

 strongest possible negative evidence that obliqna is not a Tel- 

 linomya, and though the negative evidence is thus equally 

 strong to prove that faberi is not a Tellinomya, yet the nega- 

 tive evidence was never necessary, for we had the most posi- 

 tive evidence arising from its shape to prove that it is not 

 a Tellinomya. All Tellinomya have wide denticulated hinge- 

 lines, that allowed the valves to be freely opened and spread 

 out, and held the valves in place, when the strong 

 adductor muscles closed them. No such w r ide hinge-line and 

 no such spreading of the valves could have existed in the 

 case of Palccoconcha faberi. Indeed, every indication is in 

 favor of the supposition that Palccoconcha faberi had a very 

 thin shell, with very little pearly matter, rarely capable of 

 preservation, and that the valves were held together by an 

 external ligament, and could be opened only a short distance 

 by the muscles, but never spread wide as in Tellinomya. We 

 have now, however, a fine valve of Palccoconcha faberi, with 

 the hinge-line well preserved. The hinge-line is linear, with- 

 out any thickening, except immediately below the beak. 

 There are not the slightest denticulations, crenulations. or 

 irregularities on the hinge-line. It is absolutely edentulous. 

 The ligament was external, and the beaks projected beyond 

 the ligament. 



Having thus determined the necessary generic characters 

 of Palccoconcha faberi, it will be seen that all the probabilities 

 are that obliqua possessed a thin shell, rarely, if ever, capable 

 of preservation, and that the valves were held together exclu- 



