208 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Ptychoparia (Crepicephalus) iowensis (Owen) Walcott, 1884, Bull. U. S. 

 Geol. Surv., No. 10, p. 36, pi. 6, figs. 2, 20. (Refers to use of Crepi- 

 cephalus as a subgenus and illustrates a cranidium and pygidium.) 



Ptychoparia (Crepicephalus) iowensis (Owen) Lesley, 1889, Geol. Surv. 

 Pa., Rept. P 4, p. 832. (2 text figs, only.) 



Crepicephalus iowensis (Owen) Walcott, 1899, Monogr. U. S. Geol. Surv., 

 Vol. 32, Pt. 2, p. 459. (Refers to this species as type of genus Crepi- 

 cephalus.) 



To the description given by Hall [1863] we may now add that 

 the thorax has 12 segments; and we are also enabled to illustrate 

 the form and general character of the entire dorsal shield, which 

 is rather closely related to that of C. texanus (pi. 30). There is 

 considerable variation in the form of the pygidium. In Owen's 

 illustration [1852, pi. lA, figs. 11, 13, 15] the postero-lateral spines 

 diverge from the median line of the axial lobe, but this divergence 

 decreases in other specimens until the sides of the pygidium are 

 nearly straight, and the ends of the spines curve inward toward the 

 median line. The degree of the divergence varies in specimens from 

 different localities, but in the collection from Menomonie there is 

 variation from those that are strongly divergent to those that are 

 diverging but slightly from the median line. 



The reasons for considering this species the type of the genus are 

 given under the description of the genus (p. 199). 



Crepicephalus iowensis is very abundant in the shaly and thin- 

 bedded sandstone of Wisconsin and eastern Minnesota. Only rarely 

 have the cephalon, thorax, and pygidium been found unbroken ; 

 there is only one example known to me in which they are united in 

 their natural position, and this is broken so that less than one-half 

 of the dorsal shield remains; this specimen is used as the base for 

 the restored figure i on plate 29. 



In all specimens known to me the test has disappeared, but from 

 the casts in fine sandstone the outer surface appears to have been 

 smooth. 



The larger cranidium has a length of 30 mm., and the one .speci- 

 men showing the length of the dorsal shield has the following 

 dimensions : 



Total length 78 mm. 



Cephalon 12.5 mm. 



Thorax 29.5 mm. 



Pygidium 14 mm. 



Crepicephalus iowensis is very abundant in the Eau Claire forma- 

 tion and the upper part of the Dresbach formation. 



