G PROCEEDINGS OF THE ACADEMY OF 



mountain limestone," being abundant at Quincy, 111., and intimating that Nor- 

 wood and rratten 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 C. multicosta. 



SPIRIGERA, (d'Orbigny,) Billings. 



Spirigera corpulenta, n. sp. Shell of medium size, extremely ventricose, 

 varying in outline from oval to orbicular-oval. Ventral 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 ; 

 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 th* shell present a regular convexity, 

 like the dorsal and ventral surfaces, and the lines of growth of the two valves, 

 diverciDg from the postero-lateral region complete the illusion of a dorsal or 

 ventral surface radiately ribbed. 



SYRIXGOTHYRIS, n. gen. 



Etymology, ivpiy?, a tube and BufU, a window. 



Shell with an elongated hinge-line. Ventral valve with a mesial sinus, a 

 very broad area, and a narrow triangular fissure closed toward the apex by an 

 external e'onvex 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. A low 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. 



