Ixxxii Introduction. 



the Urodelous and the larvae of the Anurous Amphibia ; illus- 

 trating well what is meant by the j^hrase ' a generalized type/ 

 The Dipnoi resemble the ElasmohrancMi and Ganoidei in having 

 the posterior pair of limbs placed near the anus, as in all Verte- 

 brata above the Teleostean Fishes in which a posterior pair of limbs 

 is developed. 



The second order of Fishes, that of the ElasmohrancMi, is repre- 

 sented by the Sharks, Rays, and Chimaerae. They differ from the 

 Dipnoi and Ganoidei in the following points besides those which 

 their name connotes. They never have an air-bladder, except occa- 

 sionally as a rudimentary structure ; they have no cranial, nor 

 clavicular bones ; their exoskeleton has the form of ' placoid' 

 granules, not of scales ; and in the males, accessory copulatory 

 organs are developed. They resemble the Ganoidei in the hetero- 

 cercal character of their tail ; in the formation of their caudal 

 haemal arches by costal elements ; and in the possession of several 

 rows of valves within their arterial bulb ; of a coating of transversely 

 striped muscular fibre on the exterior of this structure ; and of an 

 optic chiasma. 



The order Ganoidei is divisible into two sub-orders — the Chon- 

 drostei represented by the Sturgeons ; and the Holostei represented 

 by the Bony Pike, the Poli/pterus, and the Amia. They always 

 possess- a freely moving operculum, supported by one, as in Stur- 

 geons, or by several bony plates, and an air-bladder provided with 

 an air duct. With a few exceptions, [Polyptenis, Amia, ScapM- 

 rhynchus,) they possess an opercular gill as well as a pseudobranchia ; 

 and a blowing cavity, the remains of the first visceral cleft, may 

 exist together with, or independently of one, or other, or both of 

 these structures. The angular or round enamelled scales, whence 

 their name is taken, are ordinarily but not always present, the skin 

 being sometimes naked and sometimes developing bony plates as 

 in the Sturgeons. The extent to which ossification proceeds in 

 their axial skeleton is, as the two subordinal names above given 

 indicate, very various. The Holostei and especially the genus 

 Amia, make a considerable approximation towards the Physo- 

 stomous division of the Teleostean Fishes, by the distinct speciali- 

 zation of maxillary and intermaxillary bones, and by failing to 

 develope either the enamel on the scales, the fulcra on the fins, or 

 the gill on the operculum, so characteristic of Ganoidei generally. 



