10 THE FHESH-WATEU FISHES OF EUROPE. 



branchial, and, below this is the hypo-branchial. The uppermost segments 

 of the fourth branchial arch are known as the upper pharyngeal bones. 



It is remarkable that among' the Sturgeons, in which the head consists of 

 an enormously thick mass of cartilage, of similar texture to that which makes 

 the rest of the Sturgeon^'s skeleton, the superficial dermal scutes upon the 

 head are arranged so as to correspond in position with the bones which roof 

 in the brain-case of bony fishes. And in describing the Sturgeon group we 

 shall find it necessary to pre-suppose a familiarity with the structure of the 

 skull, so far as is implied in knowing the nomenclature of the upper bones of 

 the head, which is essentially the same for all animals. Thus, at the back ot 

 the head there is a single bone in the median line, named supra-occipital, and 

 at its outer margins a bone on each side, called the epiotic. In front of th^ 

 supra-occipital thei'e is a pair of bones, known as the parietals, and, external 

 to these, another paii', known as the squamosals. In front of the parietak 

 is the pair of frontal bones, more or less separated in front by the ethmoid 

 bone or bones. External to the frontal bones, so as to form the margins for 

 the orbit of the eye, are post-frontal bones behind and pre-frontal bones in 

 front, which fishes have in common with other cold-blooded vertebrata. 

 All these elements ai-e well developed in the dermal head-plates of the 

 Sturgeon, and there are other ossifications, some of which may possibly 

 represent the nasal bones, though the nasal apertures have no pre-maxillary 

 bones bounding them in front, as in the majority of vertebrata. 



The vertebral column is divided into an anterior, or thoracic portion, and a 

 posterior, or caudal portion. The number of vertebrae varies in genei'a and 

 species, and is, therefore, usually counted. Each vertebra 

 (Fig. 6), has a bi-concave body, with the remains of the 

 primitive cartilage, called the notochord, filling the spaces 

 between the cups. An arch above protects the spinal cord, 

 and in fishes of deep body is prolonged to a great height. 



Ribs are attached to the vertebrae in the abdominal 

 region, and these often possess complex accessory pro- 

 cesses (Fig. 0). 



The caudal region has a lower arch to each vertebrae, 

 %;^ corresponding to the upper neural arch. The termiua- 



Fig. G.— YEKTEniiA OF ^ion of thc tail is commonly directed upward, as in 

 3-?, rib ; i-/f(, nppcndnge of Sharks and the sturgeon, but the bony processes at- 

 tached to it, as may be seen in the Salmon, attain a 

 symmetrical development, so as to make an evenly-lobed, or homocercal fin. 



The structure of the bones which support the pectoral and ventral fins 

 is full of interest in their correspondence with the similar arches of land 



