INVERTEBRATES. 299 



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. 

 .Doroftj' 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 extremity; area very much 

 contracted, triangular, more or less arched, and very obscurely 

 defined; foramen rather large, or occupying three-fourths of 

 the small area, having nearly the form of an equilateral tri- 

 angle and apparently always open to the beak. Surface nearly 

 smooth, or only having obscure marks of growth, and some- 

 times showing, by the aid of a lens, faint traces of a radiating 

 striae. Length of largest specimen, 0.90 inch ; breadth, 0.86 

 inch; convexity, 0.60 inch; length of hinge, 0.45 inch. 



This shell agrees so nearly with some varieties of Spirifer glaber, Martin 

 (sp.), that we have not heen able to fully satisfy ourselves that it is specifically 

 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*, of 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 varieties of S. 

 glaber we have seen figured, in having a much smaller and more obscurely de- 

 fined 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 consider our shell distinct 

 from S- glaber, were that species not known to be so extremely variable. 



The specimen we have figured is the largest we have seen, the smaller ones 

 being more compressed and more regularly rounded in outline, though none of 

 them are so transverse as the prevailing forms of Martin's species, figured by 

 Mr. Davidson and others. 



Locality and position: Chester, and in Pope county, Illinois; Chester group 

 of the Lower Carboniferous series. 



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