THE SKELETON OF THE CODFISH. CAUDAL FIN. 87 



The caudal fin consists of a series of about forty-three 

 rays which radiate from the posterior end of the vertebral 

 column, being connected with the urostyle or hypural bone, 

 and with the posterior neural and haemal spines without the 

 intervention of interspinous bones. Like the other fin-rays 

 those forming the caudal fin are transversely jointed, and are 

 widened and frayed out distally. 



The tail -fin in the Cod is homocercal, i.e. it appears 

 to be symmetrically developed round the posterior end of 

 the vertebral column, though in reality a much greater pro- 

 portion is attached below the end of the vertebral column than 

 above it. It is a masked heterocercal tail. 



THE SKULL. 



Owing to the fact that very little cartilage remains in 

 the skull of the adult Codfish, its relation to the completely 

 cartilaginous skull of the Dogfish is not easily seen. Before 

 describing it therefore, the skull of the Salmon will be de- 

 scribed, as it forms an intermediate type. 



THE SKULL OF THE SALMON 1 . 



The Salmon's skull consists of (1) the chondrocranium, 

 which remains partly cartilaginous and is partly converted 

 into cartilage bone, especially in the occipital region, (2) a 

 large series of plate-like membrane bones. 



THE CHONDROCRANIUM. 



This is an elongated structure, wide behind owing to the 

 fusion of the large auditory capsules with the cranium, and 

 elongated and tapering considerably in front ; in the middle 

 it is much contracted by the large orbital cavities. 



DORSAL SURFACE OF THE CRANIUM. 



In the centre of the posterior end of the dorsal surface 

 is the supra- occipital (fig. 9, A, 1) with a prominent posterior 



1 See W. K. Parker and G. T. Bettany, The Morphology of the Skull, 

 London, 1877, chap. 3. 



