JURASSIC AMMONITES FROM EOIIIZEN & NAGATO. 17 



The shell is flatly discoidal, and widely umbilicated, the 

 umbilicus occupying about I of the shell-diameter. The whorls 

 are only slightly convex on the Lateral sides, while rounded on 

 the external side or venter, with the umbilical edge obliquely 

 sloping to the umbilical suture. Involution about I or a, little 

 less. The transverse ribs which rise at the umbilical suture are 

 numerous, fine, elevated, radial, somewhat flexuous, regularly 

 bifurcating in the inner whorls at about § their lengths, often 

 remaining simple in the body-whorl. In this latter case, a short 

 loose rib may be intercalated between two long simple ones, 

 beginning free near the place where bifurcation usually takes 

 place and going over to the venter. The number of ribs varies 

 according to age, and also according to the individuals. In a 

 shell 60 mm. in diameter (fig. 4), there are 80-90 ribs, while in 

 one 45 mm. in diameter (fig. 5) we count about 60. The inter- 

 spaces are flat and two to three times broader than the ribs 

 themselves. In a specimen shown in fig. 6 which represents 

 one-half of a shell nearly 70 mm. in diameter, the ribs are not 

 so close together as in those above mentioned. Still I believe 

 it belongs to the same species. 



This form is closely related to Dactylioccras minulatum 

 Wright {Lias Ammonites, p. 475, pi. LXXXIV., figs. 7-9) of 

 the English Upper Lias. But the latter has more numerous 

 (up to 130), and flatter straight ribs. 



