•H 6 2 M. YoKOYAMA. 



line and parallel to it ; each of the two somewhat iiiie({ual halves thus 

 formed is moderately convex, and marked by fine transverse plications, 

 some of which can become ^■ery coarse. The remaining- pcjrtion of the 

 valve is ornamented with coarse, elevated, slightly flexuous, crenated 

 ribs wh<3se number exivctly corresponds to that at' the costella} of the 

 escutcheon, being, so to speak, the continuations of the same, although 

 interrupted in their course by the intervention of the area between. 

 Tiiey arise at the border of the ;a-ea as narrow crenulated ridges, aiid 

 diverij'e in every direction, liettin"' hio-her and l)roader as thev ai)- 

 proach the palliai border, into which they pass over withcjut any marked 

 curvature. The interspaces are smooth. The palliai border is render- 

 ed dentate by these ribs. 



The internal characters of the shell are iiot well known. 



The younger specimens of this shell are a little sliorter, and the 

 ribs nujre straight and less in number (Fig. 2). 



T have already compared this species, in the work above cited, with 

 Ti'ujnuia alifvriuis Parli. It is nearer t<j the varietv called altcnuata 

 by J^ycett (A Monograph (jf the British Fossil Trigonia,''' Xo. o, p. 117, 

 pi. XXA , Fig. 6) than to its typical form. v>t\\\ there are marked 

 ditferences between the two. The most striking lies in the ribs which, 

 in the Eiii>lish form, are not onlv more numerous, but also describe 

 concentric curves in the anterior portion of the shell, whereas in 

 the fbipanese, although somewliat fiexuous in themselves, they all 

 piss over straight to the palliai border without making any distinct 

 curvature, lîesides, in the former, the marginal ends of the costelliii 

 of the escutcheon are directed posteriorly instead of anteriorly. 



A species called Tri'j mia Fi>rbesii Lycrtl (1. c. p. 122) from 

 ^'erdachellum in Iridia seems to show similarity in the course 



1) I'aheontofinijjhical Societi/, Vol. A' A' T'A', isaiied for 1875. 



