396 Transactions. 



upper part of the process, and this ridge becomes still more accentu- 

 ated in. the Patagonian late Tertiary species, including P. tehuelca, the 

 genotype, and in the most primitive species of Neoihyris in New Zealand, 

 while at the same time the process gains still further in length and height. 

 Finally, in N. lenticularis the process is so large that it fills the whole of 

 the hinge-trough, and almost hides the bifurcation of the septum ; the 

 posterior median ridge is strongly marked, while the two lateral ridges 

 bounding the hollow facet at the bottom of the posterior part of the process 

 are greatly developed, and incurved so that they nearly meet to form a 

 tube. 



The great resemblance in hinge characters between Terebratella dorsata 

 and Magellania flavescens on the one hand, and between Pachymagas tehuelca 

 and Neothyris lenticularis on the other, makes it certain that M . flavescens 

 had an ancestral form with Terebratelliform loop and Terebratelliform hinge 

 characters, while N. lenticularis had an ancestor with Terebratelliform loop 

 and Pachymagoid hinge characters. Corroborative evidence would be fur- 

 nished by a study of the hinge characters of the young stages of these 

 species, sufficient material for which I possess only in the case of the former 

 stock. 



In M . flavescens, T. sanguinea, and T. rubicunda, Terebratelliform hinge 

 characters can be recognized not only in specimens with Terebratelliform 

 loops, but in specimens with Magaselliform loops. Young specimens of 

 fossil Pachymagas also possess Pachymagoid hinge characters at a stage 

 when the loop is Magaselliform. We are thus in a position to recognize 

 two stocks, typified in their highest development by M . flavescens and N. 

 lenticularis, and to state that the generic distinctions between Terebratella 

 and Pachymagas on the one hand, and between Magellania and Neothyris 

 on the other, are well founded. 



Magella gen. nov. 



No adult specimen with Pachymagoid hinge and Magaselliform loop has 

 yet been described, but there is in the Kakanui limestone (Oamaruian) a 



species with Terebratelliform hinge cha- 

 racters and Magaselliform loop- — viz., 

 Terebratella hahanuiensis Thomson* non 

 Hutton. This species I now rename 

 Magella carinata,\ and make it the type 

 of a new genus Magella, the essential 

 characters being those already indicated. 

 Fig. Z.—MayeUa carinata Thorn- I n ld specimens the loop is rather more 



son, Kakanui limestone. In- advanced, and the septum correspondingly 

 tenor of dorsal valve, end , . , . , £ . •. - 1 , 9 J 



view, showing loop touching lower > tnan in most species placed under 

 septum. Magasella ; but the pattern is still Maga- 



selliform and not Terebratelliform, as fig. 3 

 shows, and the septum is still much higher in front than behind. The 

 deltidial plates are discrete, as in the young of Terebratella and Magellania. 

 There are probably a large number of described species which may be 

 transferred to Magella. In some with Terebratelliform hinge the loop 

 is more primitive, being in the stage called by Fischer and Oehlert 



* Trans. N.Z. Inst,, vol. 40 (1908), p. 102, pi. xiv, fig. 4, a-c. 



[" The holotype of this species is the specimen figured by me in 1908, and now in 

 the Otago Museum. 



