NATURAL SCIENCES OF PHILADELPHIA. 143 



ventral valve gibbous, with a moderately shallow sinus, which usually be- 

 comes obsolete on the posterior part of the shell, before reaching the beak. 

 Cardinal line scarcely as long as the entire width of the shell. Auriculate 

 expansions moderately developed, and ornamented with a row of short spines, 

 extending from the beak to the lateral angles, while there are indications of 

 similar spines, scattered at irregular intervals upon the lateral borders and 

 front of the shell. Dorsal valve concave, flat on the central and pos- 

 terior portions of the shell, with a slightly rounded elevation, which corres- 

 ponds to the sinus in the ventral valve. Surface of both valves ornamented 

 with rather coarse rounded strire, which increase by intercalation and bifurca- 

 tion, and are about as wide as, or a little wider than, the spaces between them. 

 These strife are more tortuous and irregular upon the dorsal than on the ven- 

 tral valve. Beak short, depressed, and extending but little beyond the car- 

 dinal border. 



Length of an average sized specimen 3-75 inches, width 4*25 inches, height 

 1-50 inch. 



This is the largest Productus knowu to us in the rocks of this country, and 

 may be distinguished from the large varieties of P. semireticulatus, which it 

 most nearly resembles, by its shorter and less elevated beak, as well as by the 

 arrangement of the spines upon its surface, and by well marked internal dif- 

 ferences which can only be explained by the aid of figures. It has heretofore 

 been referred to the European P. giganteus (Martin,) but Mr. Thos. Davidson, 

 to whom we sent specimens, has decided that it is not identical with that 

 species. 



Locality and position. Monroe County, 111., and St. Genevieve County, Mo., 

 iu the Keokuk Limestone. 



Spirifera glabra, var. contracta. Shell rather under medium size, quad- 

 rato-subcircular in outline, becoming moderately gibbous with age ; length 

 and breadth nearly equal, sides rounded ; hinge short or scarcely equalling 

 half the breadth of the valves near the middle. Dorsal valve much more 

 compressed than the other, most convex along the middle from near the 

 beak to the front, and sloping towards the sides ; hinge margin truncated ; 

 beak very small, scarcely projecting beyond the hinge line, slightly incurved ; 

 area narrow. Dorsal valve gibbous, provided with a narrow, shallow sinus, 

 commencing near the middle, and widening to the front, which is a little 

 produced to fill a shallow sub-semicircular sinus in the anterior margin of the 

 opposite valve ; beak prominent, incurved, and rather pointed at the ex- 

 tremity ; area very much contracted, triangular, more or less arched, and 

 very obscurely defined ; foramen -rather large, or occupying near three- 

 fourths of the small area, having nearly the form of an equilateral triangle, 

 and apparently open to the beak. Surface nearly smooth, or only having ob- 

 scure marks of growth, and sometimes showing, by the aid of a lens, faint 

 traces of fine radiating stride. 



Length of largest specimen, 0-86 inch; breadth 0*90 inch; convexity 0*58 

 inch ; length of hinge 0'45 inch. 



This shell agrees so nearly with some varieties of Spirifera glabra, Martin, 

 (sp.), that we have not been able to fully satisfy ourselves that it is spe- 

 cifically distinct, though we strongly suspect that it will prove to be so. In 

 form it is almost exactly like Mr. Davidson's fig. 33, pi, 1*, in his Monograph 

 of the Carboniferous Brachiopoda of Scotland, representing a rather small 

 specimen of Martin's species. It differs, however, from this, and all the varie- 

 ties of S. glabra we have seen figured, in having a much smaller, and more 

 obscurely defined ventral area. Indeed the sides of the beak of its ventral 

 valve round in so regularly to the foramen, that it is often difficult to see 

 where the margin of the area is. As this character is persistent in the five 

 specimens of different ages that we have seen, we should not hesitate to con- 



18GL] 



