130 ORDERS OF FISHES. 



The ventral fins are generally wanting, and the air-bladder 

 is destitute of a duct. 



This sub-order includes the living Trunk-fislies {Ostradon- 

 tidm), File-fishes {Balistidcc), and Globe-fishes {Gymnoclontidce). 

 The fossil forms are few in number, and the earliest date from 

 the Eocene Tertiary. They are chiefly noticeable for the re- 

 semblance to the true Ganoid fishes, produced by their par- 

 tially ossified endoskeleton and by their possession of ganoid 

 scales. 



Sub-order E. Lophobranchii. — This is a small and un- 

 important group, mainly characterised by the peculiar struc- 

 ture of the gills, which are arranged in little tufts upon the 

 branchial arches, instead of forming comb-like plates, as in 

 the typical Bony fishes. The endoskeleton is only partially 

 converted into bone, and the exoskeleton, by way of com- 

 pensation, consists of ganoid plates. The swim -bladder is 

 destitute of an air-duct. 



Tliis sub-order comprises the living Pipe-fishes {Syngna- 

 thidce) and Sea-horses {Hijjpocainpidcc). A few fossil forms 

 are known, dating from the Eocene Tertiary. 



Order II. Ganoidei. — The order of the Ganoid fishes 

 may be defined by the following characters : The endo- 

 slceleton is only 'partially ossified, the vertebral column mostly 

 remaining cartilaginous throughout life, especially amongst the 

 extinct forms of the Palceozoic j^eriod, in which the notochord is 

 often persistent. The skull is furnished with distinct cranial 

 hones, and the lower jaw is present. The exoskeleton is in the form 

 of ganoid scales, plates, or spines. There are usually two pairs 

 of limhs, in the form of fins, each swpported hy fin-rays. The 

 first rays of the fins are mostly in the form of strong siyines. 

 The pectoral arch has a clavicle, and the posterior limhs (ventral 

 fins) are placed close to the aims. The caudal fin is mostly 

 unsymmetriccd or " heterocercal," hut is sometimes " homocercal." 

 The swim-hladder is ahvays 2^resent, is often cclhdar, and is 

 jjrovided tvith aii air-duct. The intestine is often furnished 

 vnth a sjnral valve. The gills and opercular apparatus are 

 essentially the same as in the Bony fishes. The heart has one 

 auricle and a ventricle, and the hase of the hrarichial artery is 

 dilated into a hulhus arteriosus, vjhich is rhythmically contrac- 



