106 PALAEONTOLOGY OF KENTUCKY. 



angles, where it is a little flattened and projecting, so as to give a minute artic- 

 ulate appearance ; summit of mesial fold regularly arcuate from beak to base : 

 apex lightly incurved over the narrow, nearly vertical area. 



Surface on either side of the fold or sinus marked by from sixteen to twenty 

 plications, about four or five of which, nearest to the center, are dichotomous 

 from below the middle of their length ; plications low and rounded above and 

 flattened below middle, those towards the margin very slender ; first ten or 

 twelve ribs on each side of the fold or sinus occupy greater part of shell. The 

 entire surface is marked by delicate concentric striae, which are often crowded 

 into imbricating lamellose lines towards front of shell. In very perfect speci- 

 mens these concentric striae are papillose or fimbriated by fine radiating striae. 

 These fine surface-markings, however, are usually nearly or quite obliter- 

 ated. 



The interior of ventral valve shows but a partial thickening of the shell in 

 rostral cavity ; teeth are strong and short, dental plates spreading and margin- 

 ing the upper part of the ovate muscular area, which is broader above, and 

 sometimes very much resembles this feature in S. striatus. 



The casts preserve strong and prominent marks of the muscular impressions, 

 and sometimes vascular markings outside of the muscular area, while in other 

 individuals the papillose ovarian markings are very distinctly preserved. In 

 the cast of the dorsal valve there are sometimes strong impressions of the oc- 

 clusor muscular markings, and the apex also show the striae of the cardinal 

 muscular attachment. (Hall.) 



This species has some similarity with Spir. macrothyris, but is easily dis- 

 tinguished from this and all the other Spirifera of our strata by its extremely 

 elevated angular mesial fold and deep sinus, which are bounded by the dichoto- 

 mous plications. The size of this species is very variable; it measures from one 

 inch to two and a quarter inch in width, and from three-fourths of an inch to 

 an inch and seven-eighths in length. 



Spir. acuminata is considered by the European geologists, Dr. F. Roemer 

 and M. de Verneuil, as identical with the Spirifera cultrijugata of Europe ; 

 but Mr. Davidson, the eminent English Palaeontologist, has pointed out the 

 fact that the American species has bifurcating plications, which are not seen 

 in Roemer' s figures of the European species, and also that the European shell 

 has less plications than the American. Here again the figures are used to de- 

 termine the similarity and dissimilarity of two shells. The figures may not 

 be correct, or it is also possible that the specimen from which Mr. Roemer drew 

 his picture, like many of our species, did not show dichotomous ribs. Even 

 Mr. Conrad, who first described Spir. acuminata, did not mention the bifurcated 

 plications, showing that either his specimens did not possess them at all, or 

 only in such a faint condition that Mr. Conrad did not notice them. Mr. 



