A New Ammonite Fauna of the Lower Turanian of Mexico 233 



Shell of small size, little convex, slightly polygonal in the ambitus, thin- 

 ning toward the back and toward the front, the greatest width lying far 

 in front of the center of the shell ; slightly notched in front and truncated 

 behind. The upper face is convex, the ambitus rounded, the lower face 

 is nearly flat, a little thickened on the plastron and slightly depressed in 

 the region of the peristome. 



The impair ambulacrum lies in a relatively broad and moderately deep 

 furrow which begins in the apex and from these regularly widens toward 

 the ambitus, narrowing from there to the peristome; the furrow notches 

 the contour considerably. The zones of pores are composed of pairs of 

 small, nearly circular pores; the pores of each pair seem to be separated 

 by a slight thickening. The zone is wide and nearly smooth ; large tuber- 

 cles are missing there and only the fine granulation can be observed. The 

 zones of pores occupy less than half the furrow between the apex and the 

 ambitus. Farther on follow pores separated by larger intervals and of 

 an oblique position, arranged in such a manner that the pairs of the two 

 zones alternate. The exact number of these pairs could not be counted. 



The anterior paired ambulacra are moderately large and of lanceolate 

 form ; they lie in relatively deep furrows. The poriferous zones are broad 

 and consist of pairs of elongated, nearly equal pores ; the pores of each pair 

 are connected by a very distinct furrow. The interporiferous zone is rela- 

 tively narrow and has not quite the width of one of the poriferous zones. 



The posterior paired ambulacra are a little shorter than the anterior 

 ones. The poriferous zones consist of pairs of elongated nearly equal 

 pores; the pores of each pair are connected by a slight furrow. The in- 

 terporiferous zone is rather narrow and less wide than each of the porif- 

 erous zones. Outside of the petals the pores are smaller and the pairs 

 are separated by large intervals. 



On the whole surface of the interambulacra we observe apparently cren- 

 ulated and perforated tubercles, enclosed by narrow areas and very near 

 together; they are stronger on the anterior portion than on the posterior 

 one. In the interporiferous zones of the ambulacra of the upper face, 

 tubercles do not seem to exist. In the ambulacra of the lower face tuber- 

 cles are infrequent, while the interambulacra of the same face are densely 

 covered by tubercles, especially the plastron; they decrease in size from 

 the center toward the outside. The whole surface of the shell is covered 

 by a fine granulation. 



The impair posterior inter-ambulacrum of the upper face has a sharp 

 crest which is higher than the apicial apparatus. The anterior interam- 

 bulacra of the same face show much less pronounced crests; the lateral 

 interambulacra are flattened on the center, the flattened zone being lim- 



