i8l 



round pedicle opening-. Area is liardly recog-nizable beneath it. The dorsal 

 beak, which is less prominent, is crooked and extends beneath the ventral 

 beak. The median fold is not ver\' promiiient, Ijut is clearh' recognizable in 

 the anterior region, where tlie fold rises tjuitc abrupth' above the level of the 

 lateral portions, in correspondence with the sinual depression of the ventral 

 valve. The surface of both the shells is covered by a rather small number of 

 radial ribs, there being about lo or ii of them on either side, while 3 or 4 

 somewhat thinner ones are counted on the fold or in the sinus. The ribs on 

 the lateral parts sometimes inciease in number anteriorly by interpolating new 

 or younger ones at various distances from the beak ; but the increase of tlie 

 ribs is most remarkable in the sinus or on the folded portion in the middle of 

 the shell. Thus sometimes 5 ribs are found in these parts. The lateral and 

 the anterior margins fluctuate as the result of the coarse radial ribs and inter- 

 spaces that are in alternation with the former. Lateral ribs are about 0.8 mm. 

 wide on an average, but onl\- 0.5 mm. in the sinus. Here the interspaces are 

 much wider ; the reverse is true on the fold. There is a large number of 

 fluctuating growth lines that are ver)' prominent being convex in the inter- 

 spaces and conca\-c on the ribs. They are so pronn'nent that the surface looks 

 just as if it were provided with a great number of scales. This scaly aspect 

 is distinct over the whole surface of both the shells, but more so in the 

 anterior part. The shell structure is not punctate, but fibrous, and extra- 

 ordinarily thick. 



Dimensions : 









Length. 



Width. 



Thickness. 



Hinge-line. 



23 mm. 



25 mm. 



iS mm. 



16 mm. 



24 „ 



24 „ 



17 » 



14 „ 



26 „ 



30 „ 



20 ,. 



iS „ 



Localit}' :- — V 



'en-chao-tse, Chao-hua-h 



sien, Pao-hing-fu, prov. Sze-chuan, 



where this was collected together with a \'ariet},' which is described below. 

 Geological Age : — Middle Devonian. 



