On an Apparently New Species of Argonauta 

 from the Tertiary of Izumo. 



BY 



S. YOSHIWARA. 



Zoological Institute, Science College, Tokyo Imperial University. 



In the Journal of the Tökyö Geographical Society, Vol. VIII (1896) 

 Prof. K. Jimho has mentioned a fossil argonaut, found by Mr. J. Asai 

 at Agenokimura near the town of Matsue in Iugôri, Province Izumo. 

 The specimen consists of two individuals in the shape of casts, which 

 reduced to i of the natural size are shown in the accompanying plate 

 (Tab. V). The mother rock is a bluish-grey Neogene tuff, and this fossil 

 argonaut is of a special interest. 



Among Argonauta, numerous living species have been described, but 

 the numbers of fossil forms are extremely limited. Thus referring to 

 the works of A. Adams, Tryon, Hutton, Woodward, Martens, Zittel, 

 Steinmann, Maltzan and others, we find only two fossil species in 

 contrast to the thirteen living. Without taking into consideration the 

 soft part, Argonauta may be classified into three groups by the form of 

 the shell, which is usually the sole object of comparison in palaeontology. 



The first group shows a broad keel, a few and distant ribs and 

 grooves, both running from the umbilicus to the peripheral edge, and the 

 oval to trapezoid form of the cross-section of the whorl. The greater 

 numbers of argonauts belong to this group : 



1. A. Mans Sol., found in all warm seas except the Mediterranean. 



2. A. conradi Park., in New Nantucket and the Pacific Ocean. 



