BRACHIOPODA. 
109 
1863. Retzia, Billings. Proc. Portland Soc. Nat. Hist., vol. i, p. 114, pi. iii, fig. 11. 
1867. Rhynchospira, Hall. Palaeontology of N. Y., vol. iv, ji. 278, figs. 1-6. 
1879. Retzia, Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., p. 160, pi. xxv, figs. 13-21. 
1882. Retzia, Hall. Eleventh Ann. Kept. State Geologist Indiana, p. 302, pi. xxv, figs. 13-21. 
1888. Rhynchospira ?, Herrick. Bull. Denison University, vol. iv, p. 25, pi. iii, fig. 16. 
1889. Tremaiospira, Nettelroth. Kentucky Fossil Shells, pp. 135, 137, pl.^ xxxii, figs. 40-43. 
1889. Retzia, Beecher and Clarke. Mem. N. Y. State Mus., vol. i, No. 1, pp. 55, 61, x)l. v, figs. 1-16. 
Diagnosis. Shells elongate, retziiform ; hinge-line short and curved. Umbo 
of the pedicle-valve incurved, usually concealing the deltidium ; apex truncated 
by a circular foramen. Cardinal slopes gradual, scarcely excavated, not form¬ 
ing a false area. Deltidium triangular and flat or incurved; its lateral margins 
are sharply defined, and its surface traversed by ;i longitudinal median ridge, 
which is the line of solid coalescence of the constituent plates. The umbonal 
cavity does not contain the split deltidial tube which is present in the genera 
Retzia, Hustedia, etc. The teeth are small and well defined, and are not sup¬ 
ported by dental plates. Muscular impressions very obscure. 
In the brachial valve the hinge-plate has the general form of that in 
Trematospira, but is much less elevated. Its posterior extension is slight, ex¬ 
tending but a short distance beyond the hinge; it consists of two parts, a 
lower, which is closely appressed against the umbo, and deeply divided by a 
median cleft; and an upper, which is larger; conspicuously elevated and divided 
medially only at its margin, though the groove extends forward to the middle 
of the plate. The anterior portion is deeply concave and produced into two 
fiat lobes which form the crural bases. The entire plate rests on stout 
supports which diverge at the bottom, leaving a triangular cavity beneath, 
in which there is a short, sometimes obscure median septum. The 
spirals make from six to nine volutions, the primary lamellae being nar¬ 
row and not greatly incurved. The loop is situated behind the middle of 
these lamellae and is simple in its structure; its lateral branches narrow just 
above their origin, with a gentle posterior inclination, 
then broaden and meet at a little more than one-half 
the distance across the base of the coils, forming a 
broad, short, roof-shaped process, which is directed 
. . . Fig. 101. The loop of Aftyrac/jospira 
posteriorly and terminates in an oblique edge. formosa, Hall, (c.) 
