108 PALAEONTOLOGY OF NEW YORK. . 



thence inward to their point of union. The stem is short, making an angle 

 with the lateral branches, and is directed backward. It reaches the level of the 

 crura at a considerable distance in front of them and is there bifurcated, each 

 arm making a slight double or sigmoid curve. These arms are, however, too 

 short to reach the umbonal blades. The stem itself is continued for a short 

 distance above the point of bifurcation. The spiral ribbons make ten or eleven 

 volutions in full-grown individuals. Fimbriae are absent from both the spirals 

 and loop. 



Accepting this reconstruction of the loop as correct, this organ proves to be 

 precisely what might be anticipated in this Devonian genus ; it resembles that 

 of the preexisting genera Rhynchospira and Trematospira in its position on the 

 primary lamellte, its erect attitude, as well as in the slight twist in the ribbon 

 near its origin, while the length of the lateral branches and the terminal bifur- 

 cation are like Eumetria. From Rhynchospira the genus further differs in the 

 presence of the split tube in the umbonal cavity, which feature seems to attain 

 its highest development among these retzioid genera. There are also some 

 additional differences of secondary importance in the structure of the hinge- 

 plate in these two genera. 



Retzia Adrieni is, therefore, a type of distinct generic structure, of which, as 

 already remarked, no other representative is at present known It is quite 

 certain that the genus is not a member of the palaeozoic faunas of America, 

 so far as known, the so-called Retzias of our Devonian being mostly referable to 

 Rhynchospira. 



Genus RHYNCHOSPIRA, Hall. 1859. 



PLATE L, 



1852. Atrypa, Hall. Paleontology of N. Y., vol. ii, p. 280, pi. Ivii, fig-. 7. 



1857. Waldheimia, Hall. Tenth Ann. Kept. N. Y. State Cab. Nat. Hist., pp. 87-89. 



1859. Rhynchospira, Hall. Twelfth Ann. Rept. N. Y. State Cab. Nat. Hist., pp. 29, 30, figs. 1-6. 



1859. Rhynchospira, Hall. Palseontology of N. Y., vol. iii, pp. 213-217, 484, 485, pi. xxxvi, figs. 1, 2; 



pi. xxxvi A, iig. 1 ; pi. xcv A, figs. 1, 7-11. 



1861. Retzia, Billings. Canadian Journal, vol. vi, new ser., ji. 147, fig. 58. 



(1) 1862. Retzia, A. Winchell. Proc. Acad. Nat. Sci. Philadelphia, vol. vi, second ser., p. 406. 



1863. Rhynchospira, Hall. Sixteenth Ann. Rept. N. Y. State Cab. Nat. Hist., p. 58, figs. 12-17. 



1863. Rhynchospira, Hall. Ti-ansactions of the Albany Institute, vol. iv, p. 213. 



1863. Retzia, Billings. Geology of Canada, p. 373, fig. 395. 



