12 AKT. 20. M. YOKOYAMA. 



and are accompanied by fine sigmoidal lines which are especially 

 numerous in the valleys near the mouth. In young specimens 

 (figs. 2, 3) the general characters of the ribs are the same, but 

 their number is less, being at most 40 ; the sigmoidal striae are 

 often more distinct. 



The keel is sharp and bounded by a furrow on each side, 

 but as our specimens have all been subjected to rock-pressure, 

 its exact shape can not be made out. 



In one of the fragments of a large shell, there was found 

 an impression of aptychus (fig. 4) marked by oblique ribs between 

 which there are several oblique striations, so that it looks not un- 

 like the one figured by Wright (Lias Ammonites, pi. LXI, fig. 4). 



Our species is closely allied to a shell figured by Wright 

 as Ammonites Levisoni (Lias Ammonites, p. 438, pi. LXI, figs. 

 5, 6) which, according to Haug, is not a true Ammonites 

 Levisoni, but Ammonites borealis Seebach (Der Hannoversche 

 Jura, p. 150, pi. VIL, fig. 5), both of which occur in the 

 Upper Lias of Europe. The Japanese form, however, differs 

 from Ammonites borealis as figured by Seebach in having a 

 steep umbilical wall and more deeply incised suture-line, in 

 which latter respect it resembles Ammonites Levisoni as figured 

 by Wright in his fig. 3, pi. LXI. Anyhow it must be looked 

 upon as a species strongly akin to these European forms. 



Locality: — Nishi-Nakayania and Ishimachi; tolerably frequent. 



2. HILDOCERAS DENSICOSTATUM M. 



PI. III. Eigs. 1, 2. 



►Shell discoidal, compressed, keeled, widely umbilicated ; whorls 

 compressed, flat, scarcely involute, with umbilical edge rounded 



