DESCRIPTION OF SPECIES 273 



the surface is either smooth or marked by faint traces of fine granules. 

 The pygidium of P. sangamonensis is described by Meek and Worthen 

 as follows: 



"Pygidium semi-elliptic, slightly wider than long, and rather con- 

 vex, distinctly narrower and a little longer than the cephalic shield, 

 narrowing backwards, and narrowly rounded at the posterior extremity. 

 Mesial lobe prominent, a little flattened on each side, and narrower 

 than the lateral lobes, from which it is distinctly separated by broad 

 strong furrows; tapering gradually backwards, and terminating rather 

 abruptly near one-third its own length from the posterior margin, so 

 as to leave a broad, nearly flat, or more or less sloping, smooth border, 

 which extends along each side the whole length of the pygidium, but 

 becomes narrower anteriorly; segments of mesial lobe seventeen or 

 eighteen, well defined, rounded, and very nearly or quite straight. 

 Lateral lobes more depressed, and about one-third or one-fourth wider 

 than the mesial lobe,' rounding down rather abruptly to the lateral 

 margins; segments nine or ten, rounded, simple, and separated by 

 distinct furrows; all terminating abruptly at the inner edge of the 

 broad, smooth marginal zone. 



"Entire surface apparently very nearly smooth." 



Dimensions. Length of pygidium 14 mm., width 17.5 mm. 



Remarks. Two other closely related forms are P. missouriensis 

 and P. major, which, as suggested by Girty, may represent the same 

 species as P. sangamonensis, especially the first-mentioned form. P. 

 missouriensis is described as having eighteen segments on the mesial 

 lobe and eleven on the lateral lobes, which agrees exactly with the 

 Ohio form; no other essential differences can be noted although Shu- 

 mard gave no figures of his species. P. major is a considerably larger 

 form with twenty-two or twenty-three segments on the mesial lobe and 

 twelve or thirteen on the lateral Jobes, but these are differences which 

 can be accounted for by age; likewise, no illustrations are given of the 

 species. The form under discussion is, however, practically identical, 

 with the exception of size, with the specimen which Meek identified 

 with some doubt as P. major, 1 and which he states is smaller and some- 

 what narrower than Shumard's species. As neither P. missouriensis or 

 P. major was figured, and as the latter is described as typically 

 larger with a greater number of segments, it seems best for the pres- 

 ent to refer the Ohio form to P. sangamonensis although it is not un- 

 expected that that species is the same as P. missouriensis and possibly 

 as P. major. 



Horizon and locality. Lower Mercer limestone: Perry County, 

 Locality 35, r; Muskingum County, Locality 43, c; Licking County, 

 Localities 46 (r), Flint Ridge, 47, 49 , (c). 



Meek, F. B., U. S. Geol. Surv. Nebr., p. 238, PI. 3, Figs. 2a, c, 1872. 



