lo l-'LATFISHES (HK TKKOSOMATA) 



few forms from the Miocene of California, but these again represent comparatively 

 motlcrn types of Flatfishes.' 



As has been already noted (p. 2), Boulenfjer (1902) associated the Zeids and 

 rieuronectida* with the Hocene genus Amphistiitni in his division Zeorhombi, and 

 reganled the last-named as allied to the symmetrical ancestor of the Flatfishes. 

 Careful examination of three examples of Amphislium paradoxum, Agassiz,- all from 

 Monte Bolca. has convinced me that Regan (1910B) was correct in regarding this fish 

 as a I'ercoid, a view that is confirmed by a study of Eastman's (1914, pi. xlv) figure 

 of the second species of Aynphistiitm, A. bozziaytum, Massalongo. As Regan has 

 suggested, this genus is perhaps related to the existing Psettus or Platax, and Boulenger's 

 restoration of A. paradoxum shows several features of Psettodes or Zetis which are not 

 to be seen in the fossils. 



III. i:\'C)LL'TK)X OF ASVMMHTKV WITHIN THE OKDliK. 



.Although the body of a Flatfish exhibits some asymmetry, it is in the head region 

 that the most pronounced changes have taken place, so that it will be convenient 

 fir.st to deal with the organs of the head, and afterwards to consider the skeleton of 

 the body, the fins, scales, viscera and so on. 



The nio.st obvious peculiarity of the Heterosomata as a whole, and one which 

 serves to distinguish them from all other fishes, is the presence of both eyes on the 

 one .side of the head — the left in some species, the right in others (see p. 27). In the 

 generalised Pseltodes the eye of the blind side is on the dorsal edge of the head and 

 has a vertical range of vision (Fig. i). A similar condition is found in Atheresihes, 

 Remhardtiiis. Cletstlieiies and Parophrvs, all of the subfamily Pleuronectinae, but there 

 is reason to believe that in Reinhardtius, and possibly in the other genera also, this is 

 a secondary feature rather than a primitive one. the eyes may be separated by a 

 flat or somewhat concave space of van,'ing width, or, more generally, by a bony 

 ridge. In some forms the width of the interorbital region differs in the sexes. 



Examination of the skulF (Figs. 2, to) shows that in all Flatfishes the " upper " 

 ( = dorsal I eye is enclosed in a complete bony orbit, whereas the " lower " eye is bounded 

 on its outer side merely by the muscles connected with the jaws. Between the eyes is 

 a strong bony interorbital bar, formed mainly by the frontal bone of the ocular side,' 

 which has been displaced outwards and downwards. The broadened frontal of the 

 blind side may also share in the formation of this bar, and in such forms as Psettodes, 

 Hippoghssus and Scophthalinus sends forward a bony process for this purpose. In 

 lilyptocephalus this process is reduced to a slender rod of bone, and in Plenronectes it 

 merely forms a short pointed projection (Fig. 9). The main portion of the frontal 

 bone of the blind side, however, is on the upper (dorsal) side, that is to say, on the 

 urong side of its eye, although its relations with the praefrontal region and with the 

 other bones of the skull are perfectly normal. Traquair (1865) su.ggested that this 

 part of the frontal of the bhnd side represented a new process sent forwards to meet 

 another directed backwards from the corresponding praefrontal, the resulting bar or 

 bridge of bone, the pseudomesial bar, being regarded by him as necessary to maintain 

 the requisite stability of the cranium. This interpretation appears doubtful in view 

 of the normal relations of the bone in other respects. The interorbital septum, 



' These forms are : Evalh,'. inrdaiu. GWhen ; /T. /lomvri, Jordan ; Uomma l>riilinum. ]ov,Un . 

 i'orntrir (ih/k/iiiis (Jordan) ; and Zornrhnmhus vihger, Jordan. The published restorations show 

 a number of features wluch cannot readily be seen in the illustrations of the actual fossils. 



" See Woodward (1101, p. 434). 



^ For good figures and descriptions of the crania of various Heterosomata see Traquair (i86,s, 

 Scophthalmus, Hippottiossus, Plcuroiu-clcs) ; Cunningham (1890, Solca) ; Cole and Johnstone (I'loi, 

 I'iturnnicUs) ; Kyle {1921, Anwglossui. Bolhus. Rc-in!iardlius, Pleuronedes, ioUa, Aymphuriis). 



' The " upper " or coloured side of the fish is referred to throughout as the ocular side, the 

 "lower" or unpigniented side 



