ORDER GANOIDEI. 979 



Family PlatysomiDjE. — The Platysomidce. agree with the preced- 

 ing family in the characters of the vertebral column, fins, scales, and 

 the main cranial structure, but differ by the body becoming deep 

 and short, with an ovoid or rhomboidal contour, and also by minor 

 modifications in the cranial structure. The teeth are small, and 

 may be either sharp or obtuse. The chief difference in the struc- 

 ture of the skull consists in the deflection of its axis below that of 

 the vertebral column, instead of being continuous ; while the hyo- 

 mandibular, instead of being very oblique, becomes nearly vertical, 

 and the ethmoid is elongated. These changes cause the mouth to 

 be separated widely from the orbit, the jaws to become " prognath- 

 ous," and the gape of the mouth itself to be much reduced ; they 

 are readily apparent by contrasting the figure of Palceoniscus with 

 that of Chirodus. Dr Traquair regards this family as a group of 

 specialised forms descended from the Paltzoniscidai ; their external 

 resemblance to the Dapediidce being probably due merely to adap- 

 tation to similar conditions of life, and not indicating a real affinity. 

 In Eurynotus (fig. 917) the contour of the body is not so greatly 



Fig. 917. — Eurynotus crenatus ; from the Carboniferous of Scotland. Reduced. 

 (After Traquair.) 



removed from the Palaeoniscid type ; the pectoral fin is large, the 

 dorsal also large, and beginning above the pelvic, while the base of 

 the anal is short. This genus occurs in the Carboniferous of Scot- 

 land and Belgium. In Benedenius, of the Belgian Carboniferous, 

 the body becomes more oval, and the dorsal fin is placed more pos- 

 teriorly. More or less nearly allied to this group are Mesolepis, of 

 the British Carboniferous ; Eurysomus, of the Middle Permian of 

 England and Belgium ; and Wardichthys, of the Scottish Carbon- 

 iferous. In Chirodus (fig. 918), of the British Carboniferous, the 

 body is rhomboidal ; the dorsal and anal fins have a long base, 

 short rays, and an anterior spine ; while the pelvic fin is unknown, 

 and the pectoral small. Closely allied, again, is Chirodopsis, from 



