JURASSIC AMMONITES FROM ECHIZEN à NAGATO. 



2. PERISPHINCTES (GROSSOUVRIA) HIKII M. 

 PL I. Figs. 2, 8. 



There arc two specimens belonging to this species which 

 may be characterized as follows : 



Shell flatly discoidal, with a wide umbilicus. Whorls numer- 

 ous, very slightly involute, oval slightly higher than broad, 

 transversely ribbed. Ribs numerous, elevated, with the upper 

 vil'^L' obtuse, and showing the characters of the Ourvicosta type, 

 that is to say, rising at the umbilical suture, they arc at first 

 directed obliquely forward, but near the middle of the flanks 

 bend backward and then bifurcate, the ribs arising from bifurca- 

 tion going over straight to the venter. The division of ribs 

 usually takes place near the external margin of the whorl, but 

 sometimes a little lower. Now and then there are simple ribs 

 between the divided ones. Number of ribs 43-45. 



The larger of the two specimens shown in fig. 2 is pressed 

 flat on the stone, especially near the mouth. It measures about 

 39 mm. in diameter, that of the umbilicus being 20 mm. The 

 other specimen (fig. 3) which is better preserved shows the 

 diameter of the shell 25 mm. and that of the umbilicus 11 mm. 

 Therefore the umbilical width varies between 0.44 and 0.51. 



The suture-line is unknown. 



The hitherto described species to which the Japanese form 

 can be compared is Perisphinctes Steinmanni Parona and Bona- 

 relli (Sur la faune du cattovien inferieun de Savoie, pi. IX., 

 fig. 2) of the Italian Kelloway which, according to Siemiradzki, 

 is identical with Perisphinctes Bucharicus Nikitin (Notes sur 

 les depots jurassiques de V Himalaya et de l'Asie Centrale, pi, /., 



