FOSSIL HETEROSOMATA 9 



To summarise the above, the conclusion is reached that Psetlodes is the least 

 specialised member of the Heterosomata, and all the available anatomical evidence 

 supports the view that it has been derived from a generalised Percoid stock. Further, 

 the same evidence suggests that the Heterosomata represent a homogeneous group, 

 although it is just possible that the Soleidte and Cynoglossidae may have arisen from 

 another part of the Percoid stem. As was pointed out above, the suggestion has been 

 made that Psettodes is a modern addition to the Flatfishes, and is not related to the 

 other Heterosomata. In the present state of our knowledge it is perhaps impossible 

 to produce conclusive evidence against this view, but if, as seems Ukely on other 

 grounds, the Flatfishes have been derived from a Percoid stock, then Psettodes provides 

 just that intermediate stage which might be expected. 



The manner in which the Flattishes have evolved from their symmetrical 

 progenitors is the subject of a considerable literature, and some of the more important 

 features of this problem are considered in the section devoted to the evolution of 

 asymmetry. It seems reasonable to suppose that the original Flatfishes arose from 

 some marine Percoid ancestor which had adopted the habit of resting on one side — 

 a habit which is found to-day in such families as the Serranidee and Labridae. The 

 fact that some of the Wrasses frequently rest or sleep lying down on one side at the 

 bottom was first noticed by Mobius (1867), and afterwards confirmed by Verrill (1897, 

 p. 136) and Boulenger (1929) ; Verrill himself suggested the derivation of the Flat- 

 fishes from symmetrical fishes with this habit. " The common Tautog or Black-fish 

 (Tauloga onitis)," he writes, " has the common habit of resting upon one side, half 

 buried among gravel, or partly under stones, and is often curved in strange positions. 

 It is easy to imagine that the flounders originated from some symmetrical ancestral 

 form that acquired, like the tautog, the habit of resting upon one side, at first only 

 when sleeping, but afterwards continually, owing to the greater protection that this 

 habit and its imitative coloration afforded. The one-sided coloration and the changes 

 in the position of the eyes, etc., would gradually follow in accordance with well-known 

 laws of evolution." It seems possible that some forms may well have gradually 

 increased this resting period, until they finally came to lie on one side continuously, 

 concealed from both their prey and their natural enemies by the similarity of their 

 coloration to the ground on which they lay, merely darting out at intervals to seize 

 other fishes. Little is known of the living Psettodes, but it has been suggested that 

 its habits may be of this nature. 



II. FOSSIL HETEROSOMATA. 



The known examples of fossil Flatfishes are comparatively few, and provide no 

 evidence as to the origin of the order. The fact that such diverse forms as Bothus 

 (= Platophrys auctt.) and Solea were already in existence in the Middle Eocene is of 

 interest in indicating the antiquity of the group. 



I have examined four examples of Eobothiis minimus (Agassiz), preserved in the 

 Geological Department of the British Museum, all from the Middle Eocene of Monte 

 Bolca. Originally described as Rhombus minimus} this species was afterwards made 

 the type of a new genus by Eastman (1914, p. 328), who considered it as allied to the 

 Turbot and Brill group (Scophthalmina:). It certainly belongs to the sinistral family 

 Bothidas, but appears to be more nearly related to the existing genera Bothus and 

 Engypyosopon.^ I have also studied examples of Solea kirchbergana (Meyer), a typical 

 Soleid from the Miocene of Wurtemberg, and the type of S. eocenica. Woodward (1910), 

 from the Middle Eocene of Egypt. Jordan and his collaborators have described a 



' Agassiz (1839-42, p. 289, pi. xxxiv, fig. i). 



* Rhombus slamatini, Pauca, from the Oligocene of Roumania, should perhaps be placed in 

 this genus. 



