16 ART. 20 — M. YOKOYAMA. 



The shell is discoidal, compressed, and very widely um- 

 bilicated. The whorls are scarcely involute, laterally a little 

 convex, externally rounded, with a steep umbilical wall and 

 obtuse umbilical edge. They are transversely ribbed, the ribs 

 being numerous (about 50 in number in a shell 50 mm. in 

 diameter), radial, straight or slightly concave toward front, sharp, 

 elevated, either simple, or united in twos by a knob near the 

 venter forming the so-called " loop and button style," whence 

 they again separate into two on the venter. The intervals between 

 the ribs are wider than the ribs themselves. The proportion of 

 the shell-diameter to the umbilical width is about 5 to 3. 



The transverse section of a whorl can not be clearly made 

 out, owing to the rock-pressure to which the shell has been 

 subjected ; but as far as can be judged from several specimens, 

 it seems to have been oblong. 



The general shape of the shell and the manner in which 

 the ribs are joined bring our species close to Cceloceras fibulatum 

 Wright (p. 476, pi. LXXXV., figs. 5-1.1) of the Upper Lias, 

 from which however it seems to differ in having ribs sometimes 

 concave toward the front and the body-whorl a little lower, in 

 which latter respect it more closely resembles Cceloceras sub- 

 ar malum, Young. 



Locality : — Ishimachi and Nishi-Nakayama ; not rare. 



8. DACTYLIOCERAS HELIANTHOIDES m. 



PI. IV. Figs. 4-6. 



Several specimens of this neat Ammonite have been obtained, 

 both from Nishi-Nakayama and Ishimachi, though in a more or 

 less crushed condition. 



