66 



Geological Age :— This species has been discovered in both the Ordo- 

 vician and the Gotlandian formations. According to Bigsby's catalogue, how- 

 ever, it seems to have been found most abundantly in the " Lower stage " of 

 the Silurian formation ; it is rarest in the " Upper stage." From the associate 

 fossil the Chinese example cannot but represent the Ordovician Age. 



Clitambonitidae. 

 Orthisina, D'Orbignv. 



Orthisina cfr. hemipronites {v. Buch) Frech. 

 PI. XXII., Fig. 8. 

 19 [ I. Ortliisinn cf. hemipronites, Frech:— VoN Richthofen's China, vol. 

 v., p. 3- 



The material of the fossil here examined is a specimen which has already 

 been mentioned though rather casually by Frech in his work above quoted, 

 by the way of an addendum to the description of Orthisina squamata v. 

 P.'^HLEN. All that he says about this fossil is, " Eine andere Orthisina (O. cf. 

 hemipronites) lag mir von I-chou-fu,*' Provinz Hupei, vor, wo sie mit ILlaenus 

 und Isotelus sp. zusammen vorkommen." 



The writer, not being able to refer to any of the pictures of the species 

 by VON Buch, cannot but follow Fkech in calling the fossil by the name as- 

 signed to it by him. But it may be described here in the following way, so 

 that anybody who is able to compare this and the Russian form may find 

 what are the real relations between them. 



The shell is subequally biconvex, the ventral valve being shallower than 

 the dorsal. The former is rather evenly convex, though somewhat flattened 

 in the middle of the anterior margin. Consequently the ventral view of the 

 shell is somewhat quadrate, partly owing of course to its having lost the um- 

 bonal prolongation of the valve. On the contrary, the dorsal valve is much 

 more strongly gibbous rising quite abruptly above the auricular portion, so 

 that the shell does not look so quadrate in the dorsal, as in the ventral view. 



i) I-chou-fu (ggffl^) Frech's mispronounciation of I-chang-fu. 



