6 
PROCEEDINGS OP THE ACADEMY OP 
mountain limestone/' being abundant at Quincy, Ill., and intimating that Nor¬ 
wood and Pratten had erroneously assigned their species to the mountain lime¬ 
stone, in consequence of supposing all the Burlington rocks to belong to that 
series. The “ middle portion of the mountain limestone series," however— 
even as then understood—would be found far above the yellow sandstones at 
Burlington. Moreover, in referring C. Fischeri to these sandstones at the same 
locality, they place them “ at the base of the mountain limestone." It seems 
clear, then, that C. Logani belongs to the Burlington limestone, but that never¬ 
theless, the species described by Hall cannot be the same, and has been properly 
separated as C. Illinoiensis. The latter species, however, contrary to Mr. Wor- 
then’s opinion, occurs frequently in all the beds below the Burlington limestone 
•—having a range co-extensive with that of G. multicosta. 
SPIRIGERA, (d’Orbigny,) Billings. 
Spirigera corpulenta, n. sp.—Shell of medium size, extremely ventricose, 
varying in outline from oval to orbicular-oval. Yentral valve depressed from 
the anterior margin to the summit of the greatest gibbosity, which is two- 
thirds the distance to the beak ; anterior margin rather deeply sinuate, or very 
slightly so, sinus soon disappearing in a mere flattening of the valve, or trace¬ 
able backwards, in a narrow shallow groove, as far as the middle of the shell j 
umbonal region extremely inflated ; beak abruptly turned toward the opposite 
valve, not produced, truncate, circularly foraminated. Dorsal valve extremely 
ventricose near the anterior margin, slightly elevated in a mesial fold traceable 
to the most gibbous region, which is less than half way to the beak; surface 
depressed between this region and the beak; beak inconspicuous, covered by 
its fellow. External surface of casts strongly marked by numerous lamellose 
wrinkles of growth. 
Length -80 (100); breadth *70 (81) ; depth of both valves -58 (72). Breadth 
and depth of another specimen -75 and -68. 
The aspect of typical specimens is exceedingly unique. The great gibbosity of 
the rostral region of the ventral valve and the anterior region of the dorsal, 
causes the line of junction of the two valves to pass diagonally from the an¬ 
terior to the posterior region. The lateral edges of the two valves, moreover, 
lie in the same plane, so that the sides of the shell present a regular convexity, 
like the dorsal and ventral surfaces, and the lines of growth of the two valves, 
diverging from the postero-lateral region complete the illusion of a dorsal or 
ventral surface radiately ribbed. 
SYRINGOTHYRIS, n. gen. 
Etymology, a tube and Qvpis, a window. 
Shell with an elongated hinge-line. Yentral valve with a mesial sinus, a 
very broad area, and a narrow triangular fissure closed toward the apex by an 
external, convex pseudo-deltidium, beneath which, and diverging from it, is 
another transverse plate connecting the vertical dental lamellae, arched above, 
and beneath giving off a couple of median parallel lamellae, which are incurved 
so as to nearly join their inferior edges—thus forming a slit-bearing tube, 
which projects beyond the limits of the plate from which it orginates into the 
interior of the shell. Alow median ridge extends from the beak to the anterior 
part of the valve. Dorsal valve depressed, without area, with a distinct mesial 
fold. Shell structure fibrous. 
The elevated ventral and deficient dorsal area of this genus, not less than its 
external pseudo-deltidium, of one piece, ally it to Cyrtia, Dalman, and Skenidium , 
Hall. It is not known whether the arms were furnished with calcareous spiral 
supports, though the general aspect of the shell is that of a Spirifera. The shell 
substance is impunctate in all conditions and under high powers. 
Some difficulty exists in deciding on the homology of the transverse plate 
and fissured tube which characterize this genus. In the ventral valve of Merista, 
[Jan. A 
