DESCRIPTIONS OP SPECIES. 445 



in shape and other ascertained characters. These shells are remarkable for their 

 straight and nearly parallel dorsal and ventral outlines, the latter being- sometimes 

 slightly concave, the small size of the portion of the shell anterior to the umbones, ' 

 and the truncate posterior. The posterior edge is subrectilinear and inclined to 

 the hinge line at an angle of about 60'-'. This truncated appearance, though not 

 possessed by Meek and Worthen's figures, is noted by them as sooietimes being a 

 character of the species. Internal casts show the long cardinal tooth and the 

 large anterior muscular scar characteristic of the genus, and in addition a faint 

 posterior scar. 



A small specimen from the mouth of Marguerite Creek (station 2345) has the 

 shell partly preserved. It is considerablj^ smaller even than the examples figured, 

 but it is otherwise in agreement, save that the umbonal ridge is stronger and the con- 

 vexity relatively greater. The shell itself is distinctly ribbed. There is a sharp 

 ridge near the hinge line, the superior margin being stronglj^ beveled and nearly 

 horizontal when seen from above. Below this ridge there are three others which 

 are sharp but not very prominent, the third one being on the diagonal angulation. 

 So far as I can make out, these ridges do not extend to the interior of the thick shell. 

 In all the characters noted, these shells are closely similar to PI. suhcostatus, but they 

 are very much larger. The form from the red sandy bed above referred to, 1 at one 

 time identified with PI. tafi, partly on account of their large size, but they differ 

 from that species in being higher in proportion to their width and in having the 

 posterior outline less oblique to the subparallel superior and inferior margins. In 

 their existing preservation it can not be told whether these large shells have the 

 radiating ridges which have been observed in the smaller ones, but if so there can be 

 little doubt that the,y are distinct from PL taffi. 



Locality and horizon.— ^sm Juan region (stations 3340, 2341, 2342, 2345, 2346); 

 Rico formation. 



Pleukophorus angulatus Meek and Worthen ? 



1865. Pleurophxirusf angulatus. Meek and Worthen, Acad. Nat. Sci. Philadelphia, Proc, p. 247. 



Coal Measures: Wabash Cut-oft, near New Harmony, Ind. 

 1875. Pleurophoriisf angulatus. JMeek and Worthen, Geol. Surv. Illinois, Eept., vol. 6, p. 529, pi. 33, 

 fig. 5. 

 Upper Coal Measures: Wabash Cut-off, near New Harmony, Ind. 



This form has straight and nearly parallel dorsal and ventral margins. The 

 posterior edge is subrectilinear and inclined to the cardinal line at an angle of about 

 60°. The posterior inferior angle is rather sharply rounded. The portion of the 

 .shell anterior to the umbones occupies about one-sixth of the entire width. The 

 umbonal ridge is well marked, but no radiating ribs have been observed. The surface 

 is ornamented by fine concentric strife. The anterior muscular scar is strongly 

 marked. The width is about 17 mm. and the height about half as much. 



