which the middle one is generally larger than the lateral ones; the 

 fold of the dorsal valve by four to six bifurcating plications, the two 

 middle being most prominent. The plications on either side of the 

 fold and sinus are invariably simple, and although generally ten in 

 number, variations from eight to twelve were noted. 



Dimensions. The following measurements were taken of three 

 specimens of typical size: 



123 



Length 18 mm. 16 mm. 18 mm. 



Length of hinge line 19.5 mm. 16 mm. 19 mm. 



Maximum width below hinge line . . 20 mm. 19 mm. 20 mm. 



Remarks. As compared with S. boonensis which is more common, 

 S. opimus is much smaller, and less transverse, with the length and 

 width nearly equal; the hinge line is never extended as in the former 

 species. Each, however, has the lateral plications simple and those 

 of the fold and sinus bifurcated. 



S.opimus has generally been regarded as a synonym of Marcou's 

 species, S.rockymontanus, from the Mountain limestone of New 

 Mexico. According to Mather's interpretation of Marcou's species 

 from his study of certain forms from the Morrow formation of Arkansas 

 and Oklahoma, S.opimus is really distinct from the latter species. 

 He compares the two forms thus: 1 "This shell (Spirifer rockymon- 

 tanus) one of the most abundant in the Morrow collections, is char- 

 acterized by occasional bifurcations of the lateral plications in which it 

 agrees with the larger specimen figured by Marcou. S.opimus Hall 

 is apparently a form ordinarily smaller than adult members of this 

 species and its lateral plications are invariably simple, as inferred 

 from figures and descriptions. It is not synonymous with S. rocky- 

 montanus." 



An examination of Marcou's figures of S. rockymontanus show 

 that his species is marked with numerous, rather fine plications, 

 freely bifurcating on the sides as well as in the sinus and on the fold. 

 On the right side of the fold on the large figure of the dorsal valve, 

 there are four bifurcated plications, while on the large ventral valve 

 one of the plications is apparently three-forked and at least sixteen 

 lateral plications seem to be present. The hinge line as stated in the 

 description is shorter than the greatest width of the shell. Compared 

 with S.opimus, the contour is very similar as in both the length and 

 width are nearly equal, but the plications are finer, more numerous 

 and more freely bifurcating. It is of significance also that Hall 

 cites his species as occurring in Ohio, while that of Marcou was des- 

 cribed from a locality far distant. 



iMather, K. F., Bull. Den. Univ. Vol. XVIII, p. 184, 1915. Dr. Mather has kindly examined 

 some specimens of the form in question from Ohio, and states that they are identical with those which 

 he identified as S. opimus from the Morrow fauna of Arkansas. 



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