ZEORHOMBIFORMES 469 



epipleura, are generally present ; and some of the anterior radials 

 of the anal are usually fused to a single strong bone connected 

 with the haemal arches, as is often the case in deep-bodied fish. 

 The air-bladder is closed. On the whole, the symmetrical forms 

 resemble in structure the Berycidae ; but the Pleuronectidae be- 

 come much modified, and lose all trace of spines on the fins. 

 Owing chiefly to this fact, they were classed with the Gadidae as 

 Anacanthini by J. Miiller [306] ; but there can be no doubt that 

 they have no special affinity with the Cods. Whether the Zeidae 

 and Amphistiidae, as suggested by Thilo [436] and Bonlenger [40], 

 are really closely related to them must not be considered as defin- 

 itely established. Both the Amphistiidae and the Pleuronectidae 

 appear in Eocene deposits. 



BRANCH A. 



Symmetrical fish retaining well-developed spines in the fins, and an 

 eye-muscle canal. The gills become reduced to three and a half. 



Family ZEIDAE (Cyttidae). The mouth is^ protractile ; the spinous 

 dorsal well differentiated, but not quite separated ; the anal with a 

 detached front portion bearing from one to four spines ; the pelvics with 

 one spine and six to eight jointed rays. Very variable is the squamation, 

 which may consist of small scales joined together in transverse rows, or 

 of larger scales, sometimes with bony scutes at the base of the dorsal and 

 ventral fins and along the edge of the body (Zeus). The post-temporal 

 is forked, but fixed to the skull ; three out of four of the pectoral 

 radials rest on the perforate scapula. The vertebrae are numerous, from 

 thirty to forty-six. . 



Cyttoides, Wett. ; Oligocene, Europe. Zeus, Art., 'John Dory'; 

 warm seas ; Pliocene, Europe. Cyttus, Gthr. ; Cyttopsis, Gill ; Zenion, 

 J. and E. warm seas. 



BRANCH B. 



In which the spinous portions of the very elongated median fins become 

 reduced, and evenly continuous with the posterior soft portions (Fig. 498). 

 While the more primitive forms retain their symmetry (Amphistiidae), 

 the Pleuronectidae undergo modifications related to their habit of swim- 

 ming and resting on the sea-bottom on one side. 



Family AMPHISTIIDAE. This extinct family distinctly approaches 

 the Fiat-Fish in structure, but retains the bilateral symmetry. There are 

 twenty-four vertebrae, small spines on the dorsal and anal fins, one spine 

 and eight soft rays in the pelvic, and the scaling is normal. 



Amphistium, Ag. (Fig. 498) ; Eocene, Europe. 



Family PLEURONECTIDAE. The spines, the eye-muscle canal, and the 

 air-bladder are lost. The changes which bring about the secondary 

 asymmetry of the adult affect many organs in the body, and are effected 

 during the lifetime of every individual. The embryo Pleuronectid and 



