496 TELEOSTEI [CH. 



Armed with this knowledge Fishery Science can aid the fisherman 

 by enabling him to find the shoals of fish, and also in some cases by 

 protecting the fish against undue depletion of their numbers at 

 critical periods of their lives ; and in this way conserving the 

 "harvest of the sea." 



We now turn to consider the aberrant orders of Osteichthyes and 

 the first we shall mention is the Aetheospondyli, including the 

 single genus Lepidosteus, the bony Gar -Pike of the lakes and 

 rivers of North America. 



Order II. Aetheospondyli. 



The Aetheospondyli are Ganoids, i.e. they are covered all 

 over with bony rhomboidal scales with a shining covering of ganoin 

 and the free edges of the scales are beset with placoid denticles. 

 But there is no essential difference between the so-called scutes of 

 the South American Siluroids and these ganoid scales, so that they 

 cannot be regarded as a diagnostic character of the order. The 

 Aetheospondyli differ from the Teleostei and agree with the 

 Chondrichthyes in certain characters which must be regarded as 

 archaic, i.e. derived from the common ancestor of both groups 

 of fish. These archaic characters are (1) the testis is connected 

 with the kidney by a series of vasa efferentia and the archinephric 

 duct serves also as sperm duct, (2) the division of the heart known 

 as the con us is distinct and has several transverse rows of pocket 

 valves, (3) there is an optic chiasma containing fibres from both 

 eyes, beneath the fore-brain, (4) there is the rudiment of a spiral 

 valve on the intestine. The peculiar features of Aetheospondyli 

 concern the vertebral column and the skull. In the vertebral column 

 of the young Lepidosteus distinct dorsal and ventral intercalaries are 

 present as well as basidorsals and basiyentrals (neural and haemal 

 arch-pieces). All these arch-pieces are derived from groups of cells 

 budded from the inner walls of the myotomes, but if we consider the 

 pieces derived from any one myotome, then the dorsal intercalary 

 lies behind the basidorsal and the ventral intercalary lies in front of 

 the basiventral (Fig. 242, A). 



Now in discussing the vertebrae of Teleostei, we saw that the 

 bony ring constituting the centrum connected the basidorsal of one 

 myotome to the basiventral of the myotome in front of it. 



In Lepidosteus the main mass of the centrum is constituted by 

 a ring of bone connecting the basidorsal derived from one myotome 

 with the basiventral belonging to the myotome in front. The arches. 



