6 Dr. C. F. Liitken on the Changes of Form in Fishes 



types [Xiphias and Histiojihorus) differ from each other quite 

 as much in their youth as at a later period in the external 

 characters and the structure of the skeleton. I have already 

 shown elsewhere (Vidensk. Medd. f. d. naturh. For. 1875) 

 that of these two types the Histiophori represent the typical 

 group properly so called, or the central group, whilst the 

 Xiphke must be regarded as a divergent branch or " aber- 

 rant type," and that the species, so far as one can form an 

 opinion vipon this point from the data furnished by the litera- 

 ture, seem to be few in number, less numerous, in fact, than 

 has hitherto been supposed, but that they are almost cosmo- 

 politan in their geograpliical distribution. Probably we know 

 only five in all — namely, two Tetrapturi^ two Histiophori^ 

 and one Xiphias. Macheera^ C. & V., is in every respect a 

 ixViQ Histiophorus without ventral fins ; and we cannot help sus- 

 pecting that the asserted absence of the latter is founded upon 

 an error or due to the preparer. The small species of Histio- 

 phoriis which have been established, H. immaculatus^ Riipp., 

 and H. puIcheUus, C. & V., are evidently only young forms 

 (they measure respectively 18 and 4 inches), of no true specific 

 value ; and H. pulchellus particularly closely approaches the 

 young Histiophori of 5| to 60 millims. length, examined by 

 Dr. Giinther and by myself. For these last I refer the reader 

 to the Danish text, the most important facts relating to them 

 being already known ; but some remarks upon the young 

 Xiphia^^ and upon the characteristic differences which distin- 

 guish them from the young Histiophori^ will be necessary. 

 Thus, one of our young swordtish {Xiphias) 190 millims. 

 long, found in the stomach of an albacore, and consequently 

 imperfectly preserved, has a very slender form ; the mandible 

 is only 12 millims. shorter than the rostrum, which is convex 

 above and flat beneath, like that of a Tetrapturusj and twice 

 as broad as high ; the branchige present nothing remarkable, 

 and consequently have not yet acquired the character which 

 distinguishes those of the Xiphioids. The two jaws are well 

 armed with comparatively strong teeth. All the body (inclu- 

 ding the head and the rostrum) is clothed with non-imbricated 

 scales, which are keeled and ciliated — ihat is to say, furnished 

 with spines or teeth upon the keel. Two rows of these scales, 

 Avhicli strike one by their size, extend along the back on each 

 side of the dorsal fin, and two others along the belly on each 

 side of the anal fin. The scales are still very distinct in the 

 young Xiphias gladius 700 millims. long, in which one may 

 even easily recognize and trace the rows of large scales above 

 mentioned ; on the other hand, this covering, at least as re- 

 gards the teeth of the scales, occurs even in the youngest 



