456 



ELASMOBRANCHII 



[CH. 



3 



:-* 2 



better developed than in Cyclostomata. In the cranium the 

 parachordals and trabeculae are firmly fused together so as to form a 

 continuous plate and the pituitary fossa is reduced to a minute hole ; 

 there is a high and well-developed side wall and the roof extends a long 

 distance forward. The sense-capsules, nasal and auditory, are well 

 developed and firmly united with the cranium. The eyes are large 

 and highly developed, and the side wall of the cranium is indented to 

 make room for the spacious orbits in which the eyes move. There 

 is a considerable part of the head in front of the brain, which usually 

 also projects in front of the mouth. This is the rostrum or snout, 

 and it is supported by three cartilaginous rods, one ventral and two 



dorsal, projecting from the front end 

 of the cranium. These rods are the 

 forerunners of the ethmoidal region 

 in other forms. In many species the 

 opening of the nasal sac is connected 

 with the mouth by a groove called the 

 or o nasal groove (Fig. 226). There 

 are usually six gill-clefts and seven 

 visceral arches in Elasmobranchs. The 

 first cleft, sometimes called the spi- 

 racle, is rudimentary and in some 

 cases entirely absent. On the other 

 hand there is one family, the Noti- 

 danidae, with two extra clefts behind, 

 so that there are in all eight clefts and 

 nine visceral arches in this family. 



The first pair of visceral arches 

 form as we have seen the jaws. The 

 upper jaw is known as the palato- 

 pterygoquadrate bar, a compound 

 appellation derived from the names of 

 the bones by which it is represented in 

 the higher forms : the term is sometimes 



shortened to pterygoquadrate. The lower jaw is called Meckel's 

 cartilage or mandibular bar: in front a strong ligament, the 

 so-called ethmopalatine ligament, attaches the upper jaw to the skull. 

 The second pair of arches are spoken of as the hyoid, and this too is 

 divided into two portions, an upper, the hyomandibular, which is 

 firmly connected to the cranium just below the auditory capsule, 

 and a lower, the ceratohyal (Fig. 223). The upper jaw is 



FIG. 222. Diagram of a section 

 through the jaw of a Shark, 

 Odontaspis americanus, show- 

 ing the succession of teeth. 

 From Eeynolds. 



1. Teeth in use. 2. Teeth in 

 reserve. 3. Skin. 4. Carti- 

 lage of the jaw. 5. Encrust- 

 ing calcification of cartilage. 

 6. Connective tissue. 7. Ec- 

 toderm lining the mouth. 





