THE VERTEBRATA 227 



bony fishes, but also of such primitive forms as Thekdus 

 (Figs. 62 ; 65, a). 



The higher grade of fishes (Osteichthyes, bony fishes) 

 have an endoskeleton of bone, but they retain an exo- 

 skeleton of scales of some sort. They vary greatly 

 in shape from greatly compressed fo'rms, whose height is 

 nearly equal to their length, to long cylindrical eel-like 

 forms. The latter appear to be the senile forms of 

 a number of families, according to Dr. Smith Woodward ; 

 the former are, in part at least, highly specialized forms. 

 Most fishes have a fusiform shape, lying between these 

 two extremes. 



Three divisions may conveniently be recognized among 

 bony fishes. 



i. The Ganoidei. Though no longer admitted as 

 a natural group by zoologists, this makes a serviceable 

 category for palaeontology. 



The name refers to the possession of "ganoid " scales 

 i.e., large thick enamel- covered scales which typically 

 articulate together to form a coat of mail, though in some 

 genera these scales are greatly reduced. The tail is 

 typically heterocercal, but in some it acquires the super- 

 ficial symmetry of the " homocercal " type (Fig. 65, b). 

 Modern ganoids are confined to the Northern Hemi- 

 sphere and to the fresh waters (except that the sturgeons 

 migrate between river and sea). Fossil ganoids are found 

 from the Devonian (Middle Old Red Sandstone) on wards, 

 and in marine as well as in freshwater deposits. A 

 familiar example is Lepidotus (Jur.-Cret.), of which the 

 scales and teeth are common in the Wealden beds 



