STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 839 



(3) Segmentation complete in the types so far investigated, 

 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 oviducts and 

 kidney ducts uniting together and opening by a common pore to 

 the exterior. 



This absence of characters peculiar to the Ganoids is an indi- 

 cation 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 Vertebrate charac- 

 ters 1 , and the gulf which really separates them is very wide. 



There is again no indication of any close affinity between the 

 Dipnoi and, at any rate, existing Ganoids. 



Like the Ganoids, the Dipnoi 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 2 , 

 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 arches and supporting the fin-rays of the unpaired dorsal 

 and ventral fins, and their well-developed cerebral hemispheres, 



1 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. 



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

 of Elasmobranchs," Proc. Zool. Soc., 1881 [This edition, No. XX.]. 



