DESCRIPTION OF SPECIES 189 



gibbous above the middle, and abruptly rounding toward the sides and 

 front; mesial sinus well defined, simple above, and becoming marked 

 by three small plications in the middle and lower part; beak much ele- 

 vated and strongly incurved, covering the upper part of the large fora- 

 men; area high in the middle, slightly concave, continued to the ex- 

 tremities of the hinge line, vertically striated : foramen large, forming an 

 equilateral triangle. 



"Surface marked by from eight to ten simple abruptly elevated pli- 

 cations (on either side of the fold and sinus) 1 which are equal to the 

 spaces between, concentrically marked by strong imbricating lamellose 

 lines of growth, and, on well-preserved specimens, by finer radiating 

 and concentric striae." 



The Ohio forms are uniformly small with the hinge line equal to 

 or less than the greatest width of the shell below. The sinus of the 

 ventral valve is marked by three to five bifurcating plications, of 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 18mm. 16mm. 18mm. 



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



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



Remarks. As compared with S. boonensis, 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, how- 

 ever, 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: 2 "This shell (Spirifer rockymontanus) one of the most 

 abundant in the Morrow collections, is characterized 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 



J The words^in parentheses have been supplied by the writer. 



2 Mather, 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. 



