THE LOWER SILURIAN ROCKS. 689 



Rhytimya byr?iesi, Miller. 



Plate 56. Firs. 4 and 5. 

 Orthodesma byrnesi, Miller, 1881, Jour. Cin. Soc. Nat. Hist., vol. iv, p. 76. 



Dr. Miller's description of this species is not as complete nor as defi- 

 nite as we could desire, and his figures are not satisfactory. Add to this 

 that the type specimen has been lost and that no others are known that 

 have been strictly compared with the specimen upon which the species 

 was founded, it is evident that some doubt must necessarily attach to all 

 attempts at identifying the species. Yet I entertain considerable confi- 

 dence in the correctness of the present attempt, for the reason that 

 shortly after the appearance of Dr. Miller's description I had an oppor- 

 tunity of studying his type. I remember distinctly that I at once recog- 

 nized in it a member of Rhytimya* and one that was very near the 

 species just described as R. producta. Dr. Byrnes' specimen preserved 

 a good part of the test, but as it was exceedingly thin we may reasonably 

 assume that casts of the interior would not look much unlike testiferous 

 examples. Of course we cannot expect that they would preserve any- 

 thing of the superficial spines. 



The specimens here referred to the species are good casts of the in- 

 terior found at Richmond, Indiana, in beds believed to be exactly equiva- 

 lent to those at Weisburg, Indiana, from which Dr. Byrnes obtained his 

 specimen. If my memory is not at fault, they agree with that example 

 also in every essential character, so that the standing of the species may 

 be, in the absence of the original type, with justice transferred to them. 



Compared with R. producta, which Rhytimya byrnesi, as the species 

 should now be called, resembles perhaps more than any other, it will be 

 found to be relatively higher, especially in the posterior part where the 

 height equals about four-ninths of the entire length instead of one-third. 

 The anterior part of the shell again is not inflated, at any rate it is much 

 less so than in R. producta, the mesial sulcus is shallower, wider and 

 directed more obliquely backward, and the ventral margin less sinuate, 

 indeed, it is almost straight. Finally, the convexity of the shell is less, 

 the posterior margin is not so regularly rounded and the lunule a little 

 wider. For further comparisons the reader is referred to several of the 

 descriptions following. 



formation and Locality : Upper beds of the Cincinnati group, Weis- 

 burg and Richmond, Indiana. 



Rhytimya cehana, n. sp. 



Plate 56. Fig. 1. 



Shell large, compressed convex, elongate-subrhomboidal, with sub- 

 parallel dorsal and ventral margins, and obliquely truncate posterior 



: - This and other genera of Lamellibranchiata now being published for the first 

 time, was outlined in 1881-82 and has been in manuscript since 1883. 



44* G. O. 



