8 The Ganoids 



though perhaps Amia may be found to resemble the Teleostei 

 in this particular. 



" (4) A pronephros of the Teleostean type present in the 

 larva. 



" (5) Thalamencephalon very large and well developed. 



" (6) The ventricle in the posterior part of the cerebrum is 

 not divided behind into lateral halves, the roof of the undivided 

 part being extremely thin. 



" (7) Abdominal pores always present. 



"The great number of characters just given are amply 

 sufficient to differentiate the Ganoids as a group; but, curiously 

 enough, the only characters, amongst the whole series which 

 have been given, which can be regarded as peculiar to the Ganoids 

 are (i) the characters of the brain, and (2) the fact of the ovi- 

 ducts and kidney-ducts uniting together and opening by a 

 common pore to the exterior. 



"This absence of characters peculiar to the Ganoids is an 

 indication of how widely separated in organization are the 

 different members of this great group. 



"At the same time, the only group with which existing 

 Ganoids have close affinities is the Teleostei. The points they 

 have in common with the Elasmobranchii are merely such as- 

 are due to the fact that both retain numerous primitive verte- 

 brate characters,* and the gulf which really separates them is 

 very wide. 



"There is again no indication of any close affinity between 

 the Dipnoans and, at any rate, existing Ganoids. 



"Like the Ganoids, the Dipnoans are no doubt remnants of 

 a very primitive stock; but in the conversion of the air-bladder 

 into a true lung, the highly specialized character of their limbs, f 

 their peculiar autostylic skulls, the fact of their ventral nasal 

 openings leading directly into the mouth, their multisegmented 

 bars (interspinous bars) directly prolonged from the neural 

 and haemal and supporting the fin-rays of the unpaired dorsal 



* As instances of this we may cite (i) the spiral valve; (2) the frequent 

 presence of a spiracle; (3) the frequent presence of a communication between 

 the pericardium and the body-cavity; (4) the heterocercal tail. 



t Vide F. M. Balfour, "On the Development of the Skeleton of the Paired 

 Fins of Elasmobranchs," Proc. Zool. Soc., 1881. 



