588 



NORTH AMERICAN INDEX FOSSILS. 



lacral. Large, round, ventricose, sometimes subconical ; ambulacral 

 pore-belts straight, broad, with a simple paired row of double 

 pores on the under side; tubercles small, with median perforation 

 and crenulated. Comanchic-Cretacic. 



22. P. pondi Clark. (Fig. 192 1, ^-^.) Cretacic. 



Under surface flattened, with a slight concavity near the region 



of the mouth opening; ambulacra with six rows, and interambu- 



FiG. 1 92 1, a-d^ Diplopodia iexana: a, upper surface; b, side view; r, lower 

 surface of smaller individual, all X M > ^> ambulacral area enlarged ; e~g, Pedinopsis 

 pondi: <r, lateral, /, lower surface, both X K > Si ambulacral plates enlarged. 

 (After Clark.) 



lacra with twenty rows of tubercles at the ambitus ; tubercles 

 small, equal, crenulated and perforated ; peristomial opening small, 

 with distinct incisions. 



Austin Chalk (Niobrara) of Texas. 



