402 Transactions. 



is roomy, and only partially filled by a small cardinal process pointing 

 obliquely forward, and expanded to form an excavated trefoil above. 



In Rhizothyris the socket-ridges are thickened, and almost unite an 

 teriorly, where they are joined by a short stout unbifurcated septum. In 

 R. curiosa the cardinal process is small and similar to that of Magadina 

 browni ; in R. rhizoida it is large and swollen, and often fused with the 

 septum and socket-ridges, but in specimens where it is distinct it is 

 essentially similar to that of Magadina cumingi. 



It will thus be seen that, while retaining Bouchardiform shape, there is 

 an evolution shown in this series not only in loop characters, but also in 

 hinge characters. While the loop has evolved until in Rhizothyris it is 

 indistinguishable from that of Magellania, the hinge has remained much 

 more simple, not showing the bifurcation of the septum which is so pro- 

 minent in Neothyris and Pachymagas. 



Other Stocks. 



In the above account all known shells with Bouchardiform beak 

 characters have been discussed, and the majority of the known species of 

 Magasella have been transferred to the Magaselloid genera, Magella and 

 Magadina, while the probable existence of a third Magaselloid genera be- 

 longing to the Pachymagas-Neothyris stock has been indicated. There are 

 still numerous species of Terebratella and Magellania, sensu lato, which will 

 not fit into any of the genera above described, but their treatment must be 

 postponed until a revision of the Tertiary Brachiopods of Australia and 

 South America has been made with special regard to the hinge characters. 

 There are also certain Recent species of Magasella still unplaced, notably 

 the South Australian forms Magasella vercoi Blockman and M. exarata 

 Verco. The name Magasella may still conveniently be retained in a wide 

 sense as a temporary designation for species whose relationship is not 

 ciear. 



With regard to the relationship of the evolutionary stocks already indi- 

 cated, it is probable that Bouchardia is a retrograde genus from a forerunner 

 of Magadina, and that from the latter genus, by an anterior enlarge- 

 ment of the hinge-trough, the Magaselliform forerunner of Pachymagas has 

 evolved. From this suppositional genus, by evolution of loop characters 

 Pachymagas and Neothyris have evolved oa the one side, and by a further 

 reduction in the calcification of the hinge Magella has evolved on the 

 other side, giving rise later by evolution of the loop to Terebratella and 

 Magellania, 



APPENDIX: DESCRIPTIONS OF NEW SPECIES. 



Magadina browni sp. nov. Fig. 7, a-d. 



Description. — Shell rounded pentagonal, slightly longer than broad, 

 greatest breadth posterior to the middle, sides nearly straight, front more 

 or less tapering ; hinge-line nearly straight ; dorsal valve nearly flat, with 

 a short but often pronounced anterior sinus ; ventral valve moderately 

 convex, with a median carination ; commissures with a well-marked anterior 

 sinuation. sometimes narrow and deep, sometimes broad and shallower ; 

 beak short, not incurved, truncated obliquely by a relatively large, nearly 



