DESCRIPTION OF GENERA AND SPECIES. iBl 



Ptychaspis brizo (Walcott). 

 Plate 17, Figures 3-5. 



Dikeloccphalus (?) brizo WALCOTT, igo.s.Proc. U.S. Nat. Mus.,vol. xxix, p. 92. (Species described as 

 below.) 



This species is represented by the anterior portions of a large, moderately 

 convex cephalon, exclusive of the free cheeks. The glabella is subquadrilateral, 

 with the sides slightly incurved and the front nearly transverse ; it is marked by a 

 strong pair of posterior furrows that penetrate obliquely backward nearly to the 

 median line; a second pair incline slightly backward and penetrate to about one- 

 third the distance across; a third pair, narrow and very slightly impressed, extend 

 in at right angles to the sides a little less than one-third the distance; occipital 

 furrow well defined, with a slight, elongate, pit-like depression at the antero-lateral 

 angles of the glabella. 



Fixed cheeks very narrow, not much more than a ridge opposite the palpebral 

 lobes; palpebral lobes unknown; palpebral ridge rounded, and dividing the fixed 

 cheek into the flat posterior portion and the rather rapidly sloping frontal portion 

 that passes down into the concave frontal limb; frontal limb short, concave, and 

 bordered by a rounded, thick frontal rim. 



Surface marked by numerous, more or less irregularly placed, strong pustules, 

 except in the dorsal furrow and the concave frontal limb. 



The fragmentary specimens representing this species indicate a length for the 

 glabella of 22 mm., with a width in front of 14 mm. ; the concave frontal limb has a 

 length of 2.5 mm., and the thickened rounded rim has a length of about 1.5 mm. 

 The fixed cheek at the palpebral lobe has a width of 2 mm. 



The form of the glabella, frontal rim, and narrow fixed cheeks suggests Dikelo- 

 cephalus, but the strongly pustulose surface is not characteristic of the typical forms 

 of that genus. 



Formation and Locality. Upper Cambrian: (C38) Crystalline limestone near 

 the base of the Ch'au-mi-tien limestone [Blackwelder, 19070, p. 36 (part of first list 

 of fossils)], at Ch'au-mi-tien, Shan-tung, China. 



Collected by Eliot Blackwelder. 



Ptychaspis cacus Walcott. 

 Plate 17, Figures 10, n. 



Plycliaspis cacus WALCOTT, 1905. Proc. U. S. Nat. Mus., vol. xxix, p. 69. (Described as a new species 

 essentially as below.) 



General form of cephalon, exclusive of the free cheeks, subrhomboidal, rounded 

 in front, strongly convex. Glabella moderately convex over the posterior portion, 

 strongly convex at the frontal lobe; the posterior portion is divided into two lobes, 

 of about equal width, by the broad, rounded, transverse posterior furrow and a 

 narrow, slightly impressed anterior furrow, both of which arch slightly backward 

 toward the center; the frontal lobe is about as long as the two posterior lobes and 

 arches with uniform curve over to the dorsal furrow; it is convex but not globose; 

 it is marked about midway on each side by a short, very slightly impressed, narrow 

 furrow, which penetrates it at right angles to the dorsal furrow; occipital furrow 

 broad, strong, and arching slightly forward at the center; occipital ring about as 

 wide as the posterior lobe of the glabella, moderately convex, and arching slightly 

 forward near the center; dorsal furrow strongly defined at the sides and somewhat 

 less so in front of the glabella; a shallow pit occurs opposite the antero-lateral angle 

 of the glabella. 



