XX] FINS 481 



curved bones termed branch! ostegal rays, which are attached to 

 the ceratohyal and extend into the opercular membrane like the 

 ribs of an umbrella (Fig. 236). 



The bones of the palate are a single vomer in front bearing one 

 or more transverse rows of teeth, and behind it a single bone, the 

 parasphenoid, which underlies the base of the whole cranium, 

 and extends backwards beneath the basi-occipital bone (Fig. 236). 



We have now described the bones in a typical Teleostean skull, 

 which is by far the most complicated skull with which we shall 

 have to deal, and the reader may observe that any skull can be de- 

 scribed with comparative ease and simplicity by adopting the plan 

 which we have followed, i.e. first analysing the skull into cranium, 

 sense capsules, and visceral arches, and describing the bones which 

 ossify these, and then classifying the membrane bones under the 

 categories of roofing bones, cheek and lip bones, and palatal bones. 



Next to the skull and vertebral column of a Teleostean fish, 

 the most characteristic feature of its anatomy is illustrated by the 

 median fins. These fins have the pterygiophores or central 

 supports divided into segments and converted into bone. Small 

 distal segments termed baseosts lie in the bases of the fins them- 

 selves, and deeper segments called axonosts or fin-radials are 

 embedded in the body of the fish. The fin-radials or axonosts of 

 Teleostei are long bones, sometimes termed interspinous bones; 

 they correspond in number and arrangement with the neural and 

 haemal spines of the vertebrae opposite them, and they are con- 

 nected with these spines by ligaments. 



The tail fin or caudal fin consists of two symmetrical lobes, a 

 dorsal and a ventral, and 'is of the type termed homocercal in 

 order to distinguish it from the unequally lobed tail fin termed 

 heterocercal, which is characteristic of the Chondrichthyes. In- 

 vestigation of the development of Teleostei shows that the two 

 lobes of this homocercal tail correspond to the ventral lobe of the 

 heterocercal tail. The tail fin of a larval Teleost passes through a 

 heterocercal stage, and the notochord is bent upward and passes into 

 the upper lobe, but as development proceeds this upper lobe becomes 

 smaller and smaller and finally disappears, whilst the ventral lobe 

 increases in size and becomes divided into two lobes. The fin 

 radials of the ventral lobe coalesce with the corresponding haemal 

 spines to form one or two broad wedge-shaped bones termed the 

 hypurals, which afford a firm base for the expanded ventral lobe 

 now forming the whole of the tail fin. The dorsal and ventral (or, 

 S. & M. 31 



