124 
PALEONTOLOGY 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. Palasontology 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. 
1859. Trematospira, Hall. Twelfth Kept. N. Y. State Cab. Nat. Hist., pp. 27, 77. 
1859. Trematospira, Hall. Palaeontology of N. Y., vol. iii, pp. 207-212. 
1860. Trematospira, Hall. Thirteenth Kept. N. Y. State Cab. Nat. Hist., p. 82, pi. xxiv, fig. 3 ; 
pi. xxviiiA, figs. 1, 5. 
1860. Athyris?, Billings. Canadian Journal, vol. v, p. 282, figs. 45-47. 
1863. Trematospira, Hall. Sixteenth Kept. N. Y. State Cab. Nat. Hist., p. 54. 
1863. Rliynchonella, Billings. Geology of Canada, p. 315, fig. 322 ; p. 958, fig. 458. 
1863. Retzia, Billings. Proc. Portland Society Nat. Hist., jip. 112, 113, figs. 8-10. 
(?) 1866. Trematospira, A. Winchbll. Geol. Kept, of the Lower Peninsula of Michigan, p. 94. 
1867. Trematospira, 'Hk'Lij. 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, Nettelkoth. 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 
