172 THE ELASMOBRANCH FISHES 



The apex of the ventricle is continued anteriorly by the conus arteriosus 

 (c.a.), a short and narrow tube, the lumen of which is triangular and the walls 

 muscular. Internally the conus of the Elasmobranchs is universally provided 

 with three longitudinal rows of semilunar valves, corresponding to the faces 

 of the triangle, one row dorsal and the other two ventrolateral in position. 

 Figure 159 shows several types of valves. As a rule, in the sharks the number 

 of tiers in each longitudinal row decreases in the more highly specialized 

 forms. A somewhat generalized condition was seen in Heptanchus (fig. 150b) 

 in which four or five tiers occur, and a fairly general condition is that of 

 Isurus (fig. 159a). Much greater specialization is present in C ephaloscyllium 

 (fig. 159b), in which only two tiers of valves are present. A decrease in the 

 number of tiers does not necessarily indicate specialization in the rays, as is 

 shown by the fact that several tiers are present in Mohula (fig. 159c), which 

 is a highly specialized form. In most sharks the valves of the anterior row 

 are best developed and often cover practically the entire lining of this section 

 of the conus. The valves in the succeeding tiers usually decrease in size the 

 farther they are located posteriorly. Not infrequently the lips of the valves 

 are held in position by chordae fendineae and cliordae may also extend from 

 the fold of a valve posteriorly (fig. 159a) . Among the regular valves, further- 

 more, are often found smaller or accessory valves (see fig. 159c). 



ARTERIES 



Ventral or Ascending Aorta 



The ventral aorta {v. a., fig. 167) in the adult passes forward as a continuation 

 of the short conus arteriosus. In all the Elasmobranchs this, also, is a relatively 

 short trunk and divides anteriorly into right and left halves. The ventral 

 aorta is smaller in caliber than the conus and its walls are thinner. As in 

 Heptanchus, it gives off afferent arteries which carry the blood to the gill 

 region to be oxygenated. 



AFFEEENT AETERIES OF ADULT 



In all Elasmobranchs the hyoidean afferent {hy.af., fig. 166b) and the first 

 branchial afferent on each side {hr.af}) arise from a common trunk, this 

 being a bifurcation of the ventral aorta above mentioned. In many sharks 

 {Squalus, fig. 161; Mustelus antarcticus, fig. 166a) the last two afferents also 

 arise from a common trunk, but when this occurs the trunk is short. In some 

 forms the last two arise separately, as in Heptanchus {ap., fig. 150b) and 

 Chlamijdoselachus (fig. 160). The second branchial afferent arises as a single 

 outgrowth from the ventral aorta between the common trunk of the hyoidean 

 and first branchial afferents and that of the last two afferents in pentanchid 

 Elasmobranchs (Mnstelus, fig. 166). This is also the condition in at least one 

 of the rays, Basyatis dipterura (fig. 167). In rays in general, however, the 



