DESCRIPTION OF GENERA AND SPECIES. 179 



Genus PTYCHASPIS Hall. 

 Ptychaspis HALL, 1863, i6th Annual Report New York State Mus. Nat. Hist., p. 170. 



Ptychaspis acamus Walcott. 

 Plate 16, Figures 18, i8a. 



Plychaspis acamus WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 69. (Species described as 

 below.) 



This species is represented by specimens of the glabella and fragments of the 

 fixed cheeks. The glabella is moderately convex at the back and strongly convex 

 on the frontal lobe; it is divided by a strong, backward-arching furrow, which sepa- 

 rates the posterior portion as a transverse lobe, and the anterior as a large lobe 

 about as long as broad ; the latter is marked by two narrow, short, slightly impressed 

 furrows on the sides of the lobe ; occipital furrow strongly rounded, deep, and arching 

 forward at the center; occipital ring about the middle of the posterior lobe of the 

 glabella nearly flat, and with a small, sharp node at the center near the back margin; 

 frontal rim a rounded, narrow border in front of the deep, narrow dorsal furrow; 

 dorsal furrow narrow and deep opposite the palpebral lobe. 



Fixed cheeks rise rapidly from the dorsal furrow ; they are narrow and convex ; 

 palpebral lobes unknown; the anterior lobe of the glabella slightly overhangs the 

 dorsal furrow, which is deep and rounded. 



Surface marked with low, large pustules and very faint punctae. 



The type specimen of the cephalon in the collection has a length of n mm., 

 with a width of 6 mm. 



This species is characterized by the form of the large frontal lobe, the strong 

 transverse furrows, and narrow posterior lobe of the glabella, and its peculiar surface. 



The typical specimens of this species were referred in the original description to 

 the Middle Cambrian, Ch'ang-hia formation. Further study and comparison with 

 material from the upper part of the Upper Cambrian, Ch'au-mi-tien formation, 

 lead me to think that it is highly probable that the specimens labeled as from the 

 Ch'ang-hia formation were collected from the higher horizon of the Ch'au-mi-tien, 

 and accidentally placed with material from the Ch'ang-hia. This is rendered 

 more probable, as the two lots of specimens were collected at and near Ch'au-mi-tien. 

 The specimens from the upper part of the Ch'au-mi-tien formation appear to be 

 identical with the type, and from a lithologically similar limestone. In view of 

 this, and the fact that the genus Ptychaspis is unknown from the Ch'ang-hia forma- 

 tion, I have changed the reference to the Upper Cambrian, Ch'au-mi-tien formation. 



Formation and Locality. Upper Cambrian: (C45) Limestone about 40 feet 

 (12 m.) below the top of the Ch'au-mi-tien limestone [Blackwelder, 19070, p. 36 

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



Collected by Eliot Blackwelder. 



Ptychaspis baubo (Walcott). 

 Plate 17, Figures 2, 20. 



Dikelocephalus (?) baubo WALCOTT, 1905, Proc. U. S. Nat. Mus., vol. xxix, p. 91. (Species described 

 essentially as below.) 



The description of Ptychaspis brizo applies to this form, with the exception that 

 P. baubo has a more rounded front to the glabella, and its frontal rim and border 

 vary somewhat in form. In P. baubo the palpebral lobe is preserved, and shows it 

 to have been relatively small and short, and marked just within the rim by a rather 

 deep furrow. A cephalon of P. baubo 16 mm. in length has a glabella 12 mm. in 

 length, frontal rim and limb, 2 mm., and occipital furrow and ring, 2 mm. in length; 

 the glabella has a width of 9 mm. opposite the palpebral lobe. 



