28 THE VOYAGE OF H.M.S. CHALLENGER. 



appeared. In the specimen which I describe here, it is reduced to a short rudiment. 

 Evidently it is of use only whilst the young fish leads a free and pelagic life, and dis- 

 appears when the Fierasfer assumes the habits of a commensal. 



There cannot be much doubt as to our specimen representing a more advanced stage 

 of the young of Fierasfer acus, the resemblance of its head and of the proportions of 

 its body to young Fierasfer acus from the ]\'Iediterranean being very great. . However, 

 it should be remembered that that species has hitherto not been found so far northwards 

 as the British Channel, whilst another species of the genus, Fierasfer dentatus, is known, 

 though from two specimens only, to oecur on the Irish coast.' We possess two repre- 

 sentations of the young of this species, a very rude one by Putnam," and an excellent 

 one by Emery,^ both of which agree in showing that the young of Fierasfer dentatus is 

 a much more slender form than that of Fierasfer acus. The latter species therefore has 

 to be added to the British fauna. 



The specimen is 104 mm. long, the head measuring 5^ mm., and the distance of the 

 snout from the beginning of the anal fin 11 mm. Head and tail are much comjjressed, 

 the latter terminating in an extremely fine filament into which the vertebral elements 

 do not entei'. The eye is of moderate size, rather shorter than the snout, which is obtuse, 

 with the jaws equal in front. The maxillary extends beyond the centre of the eye and 

 the few teeth which can be observed are minute. The abdomen projects much, the vent 

 being behind and not in front of the projection. The dorsal fin commences very little 

 in advance of the anal ; of the long ray which is so remarkable a feature in very young 

 specimens, only a short rudiment remains. The fin behind the ray is at first only a low 

 ridge which becomes higher towards the middle of the length of the tail, and decreases 

 in height again behind. In its entire course it is conspicuously lower than the anal fin, 

 which, about the middle of its length, is nearly as high as the body above. 



The specimen was obtained on August 9, 1882, in the Faroe Channel, close to the 

 surface, during the cruise of H.M.S. " Triton." 



Of the two figures on Plate IV. the upper represents the specimen of the natural size, 

 the lower the anterior part of the body enlarged. 



Pleueonectid^. 



Eight young specimens were obtained in the Mid Atlantic swimming at the surface at 

 night, on August 16, 1873, and on April 18, 1876. They are 1 inch long, perfectly 

 symmetrical, and agree entirely with the " pelagic Plagusise " of the same size described 

 and figured by Steenstrup. They do not throw further light on their origin, which is 

 still obscure. 



On April 11, 1876, when sailing ofi' the coast of Sierra Leone in lat. 7° 33' N., and 



■ CoUett has descrilied another specimen from Norway, Chriatian. Vidensk Forlumdl, 1882, No. 19, c. tab. 

 - Proc. Boston Soc. Nat. Hist., 1874, vol. xvi. p. 347 (Encheliophis tenuis). 

 ^ Atti R. Accad. d. Lincei, 1879-80, vol. vii. tav. 1, a. 



