178 



SUB-CLASS GANOIDEI 



the scleroblasts of which become successively included' in the 

 scale to form the bone corpuscles. The ganoin is formed later 

 (in a fish of fifty- two months) by the cells on the upper surface of 

 the plate. The teeth are developed as in Selachians on a papilla 

 of dermis projecting into the epidermis. The cells of the papilla 

 form the dentine and those of the epidermis the small cap of 

 enamel. Some of the scales possess processes which indicate 

 where teeth were formerly attached. 



There is no spiracle in the 

 embryo or adult, though there 

 is a pouch-like trace of one in the 

 embryo ; but the hyoid bears a gill, 

 dorsal to which is a structure called 

 by J. Miiller the pseudobranch. 

 This so-called pseudobranch is not 

 found in the larva. There are 

 four double gills, and a cleft behind 

 the fourth branchial arch. 



The conus arteriosus possesses 

 eight * equally developed longi- 

 tudinal rows of valves, five in 

 each row. The ventral aorta gives 

 off on each side posteriorly two 

 branches, one to the third and 

 fourth branchial arches, and one to 

 the second ; anteriorly it also 

 gives oflf two, the posterior of 

 which goes to the first branchial 

 and the anterior to the hyoid. 

 The so-called pseudobranch ap- 

 pears to be supplied by arterial blood from the efferent 

 vessel of the first branchial arch, as in other fishes. 

 The left ductus Cuvieri opens into the sinus, the right 

 into the auricle. The stomach is without a blind sac and 

 there is a number of pyloric caeca and a small but distinct 

 pancreas. The intestine is coiled and has a spiral valve pos- 



-*%. 106. — Dorsal view of the brain 

 of Lepidosteits (after Balfour and 

 Parker), cb cerebellum ; ce, ce an- 

 terior and posterior lobe of cere- 

 brum ; m o medulla oblongata ; 

 olf olfactory lobes ; op.l optic 

 lobes ; v.th vesicle of thalamen- 

 cephalon. 



* Boa.s states, Morph. Jahrb. 6, p. 323, that the valves of four of^these 

 rows are smaller than those of the other fom-. There seems to be some 

 variation in the valves of the conus of Ganoids. 



