124 PALJEONTOLOGY OF NEW YORK. 



grooves. There is no hinge-plate spread out between the sockets. The crura 

 take their origin immediately at the apex, as thin, shelly plates sloping strongly 

 toward the middle line, and thus very nearly reach the bottom of the valve. 

 As far as the dental sockets extend, these crural plates are fastened to them. 

 Lower down they become free, and then form rather broad, shelly blades, which 

 always retain their sloping position towards the middle line of the valve. 

 They are sunk deeply into the interior of the valve, extending not very far 

 from the bottom of it." 



Shells of this structure are not as yet known in American faunas. 



Genus TREMATOSPIRA, Hall. 1859. 



PLATE XLIX. 



1852. Atrypa, Hall. Palaeontology of N. Y., vol. ii, p. 273, pi. Ivi, fig. 3. 



1857. Spirifer, Hall. Tenth Kept. N. Y. State Cab. Nat. Hist., p. 59, figs. 1-6 ; pp. 60, 168. 



1R.')9. Trematospira, Hall. Twelfth Rept. N. Y. State Cab. Nat. Hist., pp. 27, 77. 



1859. TrenuUospira, Hall. Palteontology of N. Y., vol. iii, pp. 207-212. 



1860. Tretnatospira, Hall. Thirteenth Rept. N. Y. State Cab. Nat. Hist., p. 82, pi. xxiv, fig, 3 i 



pi. xxviiiA, figs. 1, 5. 

 1860. Athyris f, Billings. Canadian Journal, vol. v, p. 282, figs. 45-47. 

 1863. Trematospira, Hall. Sixteenth Rept. N. Y. State Cab. Nat. Hist., p. 54. 

 1863. HJiynchonella, Bii.ling.s. Geology of Canada, p. 315, fig. 322 ; p. 958, fig. 458. 

 1863. Hetzia, Billikgs. Proc. Portland Society Nat. Hist., pp. 112, 113, figs. 8-10. 

 (?) 1866. Treinatospira, A. Winchkll. Geol. Rept. of the Lower Peninsula of Michigan, p. 94. 



1867. Trematospira, Hkhh. Palaeontology of N. Y., vol. iv, pp. 271, 272, 276, figs. 1-6; pi. xlv, 



figs. 7-15. 

 1884. Trematospira, Walcott. Monogr. U. S. Geol. Survey, vol. viii, p. 151, pi. iv, fig. 3. 

 1889. Trematospira, Nkttblroth. Kentucky Fossil Shells, pp. 135, 136, pi. xvi, figs. 15-19. 



Diagnosis. Shells transverse, subequally convex, with median fold and sinus 

 on brachial and pedicle-valves respectively. Surface covered with radial, 

 coarse or fine, simple or duplicate plications. Hinge-line straight, often long ; 

 cardinal extremities abruptly rounded ; anterior margin sinuate. Umbo of the 

 pedicle-valve incurved, its apex truncated by a circular foramen. Beneath it 

 lies the delthyrium, which is covered by two short incurved plates, more or 

 less closely anchylosed along the median suture, and so greatly thickened on 

 their interior surface as to appear continuous with the substance of the valve. 



This deltidial plate does not extend downward much more than one-half the 

 distance from the apex to the cardinal margin, leaving beneath it a crescentic 

 opening which is occupied by the beak of the opposite valve. On either side 



