SPIRIFERA. 219 



Retzia ? carbonaria, Dav. Plate LI, fig. 3. 



Spec. Ckar. Shell ovate, longer than wide ; valves moderately and equally convex, 

 without fold or sinus, margin of valves nearly straight ; beak moderately produced, 

 incurved and truncated by a circular foramen, slightly separated from the hinge-line by a 

 small deltidium. Surface marked with about twenty-four small rounded ribs ; shell 

 structure minutely punctured. Interior unknown. 



Length 9^, width 7|, depth 5^ lines. 



Obs. A single example of this interesting species only has been hitherto discovered, 

 and it was not until after much research and hesitation that I venture to apply to it a new 

 specific denomination. Prof. De Koninck, to whom I submitted drawings, pronounced 

 it quite distinct from Retzia serpentina, in which the strise are much more numerous. At 

 one time I thought it might perhaps be referred to Terebratula Marcyi, of Shumard 

 (' Palaeontology of the Red River of Louisiana,' PI. I, figs. 4 and 6), as the description given 

 of that shell by the American author, so nearly agreed with that of our British fossil, but 

 having forwarded drawings of our specimen to Prof. Hall, that distinguished palaeon- 

 tologist seemed inclined to consider the English shell as belonging to a more robust species 

 with smaller beak and fewer ribs, and that it differs likewise from Retzia vera, in several 

 particulars. 



Some uncertainty as to the genus to which it belongs must naturally prevail, since 

 we are unacquainted with its interior dispositions. I am also undecided whether it 

 belongs to Retzia or to Prof. Hall's sub-genus Rhynchospira} proposed for several 

 shells which bear a close resemblance both in general form and in the interior spires to 

 Retzia ; but of which the dorsal valve never presents the straight extended hinge-line, 

 nor the ventral valve the short area common to all true species of that genus. 



I am indebted to my friend Mr. Salter for the first knowledge of this new British 

 fossil, which was obtained from the lower Carboniferous Shales of Skrinkle, Pem- 

 brokeshire, and which is now preserved in the Museum of Practical Geology. 



Spirifera. — Since describing the Spirifers, many more specimens and observa- 

 tions have been gradually assembled, which will necessitate the introduction of some 

 alterations and additions to what has been written upon the subject. Thirty-seven species 

 are described in pages 19 to 76 of our Monograph, but it must be remembered that some 

 of these were at the time doubtfully and provisionally retained from want of sufficient 

 grounds for rejection or adoption, and that it was only during the interval that uncertainty 

 has been dispelled in certain cases, while a few species that had been supposed distinct 

 subsequently proved to be varieties of some of the others. In the following list a point of 



1 'Twelfth Annual Report of the Regents of the University of the State of New York,' No. 185, p. 29, 

 1859. 



29 



