[burwash] NE^^■ SPECIES OF MARINE INVERTEBRATES 89 



Rhynchonella magnicostata (sp. nov.) 



(Plate III, Fig. 9, a, b, c) 



Cf. R. gnathophora* from which, however, it seems to be quite 

 distinct. The species has also some similarity to R. plicatilis (Sowerby). 



The general outline is trigonal, and the maximum width lies about 

 one-third of the length from the anterior margin measured toward 

 the beaks of the pedicle valve. The lateral margins are more narrowly 

 rounded than the anterior, which extends in a broad curve from side 

 to side. The posterolateral margins are slightly concave and diverge 

 from the beaks to a point nearly opposite the middle of the mesial 

 line of the shell. The beak of the pedicle valve is not well shewn in 

 the material collected, but appears to have been fairly prominent. 

 The cardinal area extends about half way from the beaks to the ex- 

 tremities of the posterolateral margins. One of the specimens has a 

 fairly well-marked sinus involving four plications and extending about 

 half the length of the shell, and a corresponding mesial ridge on the 

 brachial valve on which are five plications. The total number of 

 plications is about twelve in each valve. Both valves are marked by 

 fine concentric growth-lines, which recede toward the beaks in crossing 

 the plications, and develop a tendency to rugosity toward the anterior 

 margins. One or two much stronger growth-lines cross the valves 

 between a third and a half the distance from the beaks to the anterior 

 margin, and enclose an area betwen them and the beaks in which 

 the plications are much less distinct than further forward. 



*Pal. Cal., Vol. 1, p. 39, Plate 8. 



