Holt and Calderwood — Report on the Rarer Fishes. 367 



Why such a form as Phyeis Uennioides or Pristiurus melanostoma should be able 

 to compete on equal terms with the littoral fishes of the Lusitanian region, and 

 should fail with those which inhabit our own shallow waters is not apparent ; but 

 we certainly think that competition, not temperature, has consigned them to the 

 deeper regions of our own sea, there to propagate, it may be, a race of typically 

 abysmal descendants. 



It is obviously impossible to formulate a rule which shall be applicable to all 

 species alike, since we find that the conditions of latitude (and of competition) 

 which admit a comparatively bathybial species, to littoral waters, are actually 

 those under which another species (able to maintain its littoral position wher- 

 ever it is found) reaches its greatest size. Thus the Witch {Rh. megastoma) 

 inhabits only the shallow waters of the Iceland coast instead of the surrounding 

 depths, while, under similar conditions of life, the Plaice {PI. platessa) attains its 

 greatest known size. 



Indeed, in considering these matters, it is evident that we must take into 

 account, not one condition or series of conditions (whether physical, competitive, 

 or otherwise), but every condition which can possibly afPect the species; and for 

 this purpose the necessary information is lacking, and may, perhaps, never be 

 accumulated. 



It is, therefore, with a full consciousness of the inadequacy of our knowledge 

 and of the probable imperfection of our conclusions that we have ventured to 

 make the above suggestions. We may recajjitulate them thus : that though, no 

 doubt, there is a degree (whether of cold or heat) beyond which fish of a given 

 species cannot exist, the cause that governs a change of habitat in accordance 

 with latitude is mainly one of competition, and not of temperatui-e. 



In concluding our introductory statement, a few words are necessary as to the 

 arrangement and classification of the forms dealt with. In selecting species for 

 illustration we have sought to figure such as have not already been depicted in 

 works dealing with British or Irish zoology, since our treatment of the material 

 is, in a sense, national, in accordance with the wishes of the Society's Council. 

 It follows that several forms, of which good drawings already exist, either in 

 works of general zoology, such as the "'Challenger' Memoir," or in foreign 

 literature, are again figured here. We have also endeavoured, by the insertion of 

 generic diagnosis, where required, to identify fish belonging to genera not already 

 described in the literature of British ichthyology, and whenever we have found no 

 reason for alteration, we have simply reproduced the diagnosis of previous 

 authors, with due acknowledgment. It will be noticed that the classification 

 we have employed differs from that adopted by one of us in the reports 

 published in the "Scientific Proceedings" (vol. vii., N. S.. Pt. iv.). In these 

 reports, which are more or less popular in character, it seemed advisable to deal 



