392 J. A. Thomson — Recent and Tertiary Rhynchonellids. 



In JTemithyris there is a small ridge separating the muscular 

 impressions of the dorsal valve which might be described as a 

 rudimentary septum. It does not reach posteriorly to the cardinalia. 

 The latter consists of a pair of socket-ridges which spring from the 

 sides of tbe valve and project obliquely inwards and upwards, at 

 the same time converging posteriorly nearly to a point. They bear 

 the crural bases on their upper inner margin, fused with them for 

 their whole length. There is no hinge-plate. The cardinal process 

 is transverse and deeply embayed in the middle and is supported by 

 the posterior ends of the crural bases at the sides, but rests in its 

 middle part directly on the umbo of the valve. 



In Frieleia there is a short low slender mesial septum uniting with 

 the cardinalia. The latter consists of two transversely striated socket- 

 ridges diverging at an angle of about 120°, to which the crural bases 

 are attached along their inner sides. To these in turn are attached 

 two hinge-plates, excavate underneath, uniting mesially and resting 

 on the septum. These hinge-plates differ from those of Magellania in 

 possessing an embayed instead of a pointed front. On the hinge-plates 

 there is a small cardinal process. 



JEtheia resembles Frieleia in the presence of a short septum uniting 

 with the cardinalia, but differs in possessing no excavate hinge-plate. 

 The whole space between the socket-ridges from the floor of the shell 

 as far up as the crural bases is solid, forming an elevated floor, and on 

 this the cardinal process is superimposed. 



Cryptopora possesses a long septum which is high near its anterior 

 end, but rapidly lessens in height posteriorly until it unites with the 

 cardinalia. Davidson states that a hinge-plate is present, but his 

 figures do not bear this out. There is certainly a cardinal process 

 superimposed on the end of the septum. 



In Neorhynchia there is a low thread-like septum, perhaps only 

 comparable to the ridge in Hemithyris. Dall has not yet figured this 

 species and does not state whether the septum reaches the cardinalia, 

 and mentions no hinge-plate or cardinal process. 



Conclusion. 

 Owing to the small number of genera represented amongst Recent 

 and Tertiary Brachiopods it is possible to present their major differences 

 in tabular form. 



