ABT. 1. RUDISTID SHEULS FROM MEXICO STEPHENSON. 13 



bounds the inner margin of the rim. No ligamental ridge can be 

 detected, for which reason the species is classed with the genus 

 Durcmla rather than with Sauvagesia. 



The exterior of the shell is marked by about a dozen major, broadly 

 rounded, low longitudinal ribs of somewhat irregular width and by 

 two siphonal areas; each of the major ribs bears 6 or 8 minor ribs, 

 and the bottoms of the depressions between the major ribs is cut by 

 a narrow, sharply defined channel. 



The anterior siphonal area (E) is broad, flat, and scarcely exca- 

 vated and is ornamented by about a dozen minor ribs. The posterior 

 siphonal area (S) is only about half as wide as the anterior one, is 

 deeply excavated, and bears 9 or 10 narrow, distinct ribs. The longi- 

 tudinal band between the siphonal areas is also marked by minor 

 ribs and is split by a central narrow, sharp channel. Over the entire 

 outer surface the growth lines are fine, distinct, and sharply wavy 

 where they cross the minor ribs. 



Locality. — Found in an arroyo 3^ kilometers west by north of 

 Manuel station (M. G. O. Acq. No. 135), which is about 81 kilometers 

 northwest of Tampico, in the State of Tamaulipas, Mexico. 



Type.—C2ii. No. 32506, U.S.N.M. 



Geologic position. — Upper part of San Felipe formation, corre- 

 sponding in age approximately to the upper part of the type Austin 

 chalk at Austin, Texas. 



