i893 EVOLUTION OF BRACHIOPODA. 51 



found to he obviously capable of reproduction during the Magasiform 

 and the Magaselliform phases, which precede that of Terchratella, and 

 thus an inferior race of Magasello" is perpetuated which never attains 

 the characteristics of the adult Teyehratella. The authors deem it 

 advisable, therefore, to retain Ball's subgeneric name of Magasella for 

 such permanently arrested types, although it is quite clear that four- 

 fifths of the so-called species will prove, as the American authority 

 admits, to be the immature forms of different species of Tcvehratello', 

 with which genus, it is well to remember, they invariably occur asso- 

 ciated. Some Tcvehratella' also remain so throughout life, but with 

 others the terebratelliform stage is transitory, and they pass up into 

 true Magellanae. Many such grades were arrested and became stable 

 during geological periods, and should therefore retain generic value. 

 Among these the authors cite Centronella, Magas, Tevebratella, and 

 possibly Ismenia of King. 



This is a distinct advance in our knowledge of the Brachio- 

 poda, and throws some light on the origin of genera. It illustrates 

 at the same time the difficulties under which palaeontologists labour 

 in describing and naming obscure fossil forms, deprived of the 

 assistance to be derived from a study of the developmental history 

 of any member of the group in the living condition. The simultaneous 

 occurrence, so often noticed, of a smooth series and of ribbed 

 examples of the same species, indicate the futility of relying on these 

 mutable external features as indicators of species. The most diver- 

 gent views are taken of generic values ; for instance, Mr. H. Douville 

 proposed his genus Neothyris for Waldheimia lenticulavis, a well-known 

 form occurring in the New Zealand province, both in the living and fossil 

 condition, allied to the W. kergueleneiisis, and so closely related to 

 the giant W. venosa of the Magellanian province, that Dall even con- 

 siders it as a varietal form or Novo-Zealandian race of the S. American 

 species. " The difference in colour seems the main distinction " {4). 

 Qihlert's sectional employment of the term Neothyrine as differen- 

 tiating the smooth Magellan:^ like venosa, and leiificitlaris, from the 

 pleated forms, such as M. flavescens, is far less objectionable. 



Now that Terehvatulina wyvilhi has been definitely promoted, and 

 with good reason, to be the type of a new family, the Dyscoliidae. 

 in which the short united " arms " resemble a freemason's apron, 

 the fine T. crossii from Japanese waters is the largest known 

 species of that sub-genus. It is remarkable that it should 

 have been also dredged by the " Romanche " off the coast of Pata- 

 gonia. Liothyris vwseleyi, first obtained by the " Challenger " off 

 Kerguelen islands, and subsequently by Dall in the West Indies 

 (Blake Expedition), also occurs off Tierra del Fuego, and proves to 

 be a well-defined, far-ranging species of the widely-distributed Lio- 

 thyrine group of Terebratula. 



The Magellanian province, to which the Marion isles, the 

 Crozets and Prince Edward's Island are now attached by Gihlert and 



£2 



