6 ART. 6. — M. YOKOYAMA OX SOME 



be called weakly falcate, as they curve a little backward after 

 bifurcation and then bend strongly forward near the external 

 keel. 



This form closely resembles the IMiddle Liassic species 

 Harpoceras lytlieiiae Youxg, but it is at once distinguished from 

 it by its bifurcating ribs, in which respect it is not unlike some 

 of the many varieties of Harpoceras aalense Zieten. 



A single cast of an external surface of the shell was obtain- 

 ed in a sandy clayslate of Hosoura, our ligure having been drawn 

 from a plaster cast of it. 



3. LYTOCERAS cf. LINEATUM Schloth. 

 PL II. Fig. Ö. 



An external impression of a Lytoceras found in a clayslate 

 of Aratohama, though water-worn and imperfect, shows characters 

 so peculiar to the group of Lytocevaa fi)iibrlalum Sow. of the Lias, 

 that I do not hesitate to bring it under that group, and indeed 

 very near to Lytoceras Imeaiimi Schlotii. The last volution of 

 the shell measuring about 70 mm. in diameter is provided with 

 many fine ribs which at certain intervals arc coarser and decidedly 

 fimbriate. The fimbriation of the ribs becomes more frequent as 

 we approach the shell-mouth near which almost every rib is 

 fimbriate. The non-fimbriate ribs arc very fine, thread-like, more 

 or less straight, and in some cases showing a tendency to 

 fimbriation. 



Besides those above mentioned, there is a single wall-like 

 elevated rib near the middle of the last half of the last whorl. 



