194 BULLETIN OF THE 



from which at about the middle it may be separated. This plate reaches 

 cephalad to the middle of the conus arteriosus, but is not bound to it in 

 any way. 



The conus arteriosus (see Figure 2) forms a thick spindle-shaped 

 trunk about an inch long and one fourth of an inch in diameter. It is 

 provided with six rows of valves, all of which are quite small, except the 

 anterior set of three, which are large, tridentate, and formed of a white 

 tough tissue of a cartilaginous consistency. In Odontaspis the conus is 

 bounded anteriorly by three large valves, each of which consists of two 

 thin membranes united by a thick median bar which ends in a pointed 

 projection beyond the anterior border of the valve. 



The pylangium terminates anteriorly in a synangium or bulbus arte- 

 riosus, from which spring three vessels, one median and two lateral, the 

 ventral aorta, and the sixth pair of afferent branchial arteries, respect- 

 ively. The synangium is not so well developed in Chlamydoselachus as 

 in Raja or most Elasmobranchs, for, while in the former a single pair of 

 afferent branchial arteries arise from it, in the latter the common trunk 

 known as the posterior innominate artery of the skate represents at least 

 three primitive afferent branchial vessels, and in consequence the synan- 

 gium represents a very much greater portion of the primitive ventral 

 aorta in Eaja than it does in Chlamydoselachus. 



The synangium of Chlamydoselachus includes the enlarged end of 

 the ventral aorta lying between the last pair of pylangial valves and 

 the point of origin of the ventral aorta (in the restricted sense), and 

 the sixth pair of afferent branchial arteries. It forms a truncated cone- 

 shaped body, the homologue of the bulbus arteriosus of bony fishes. 

 The ventral aorta continues the synangium forwards along the median 

 line, and lies in a distinct sheath formed by the connective tissue sepa- 

 rating the basibranchial cartilages from the muscles of the floor of the 

 branchial basket. This sheath forms the outer wall of a lymph space. 



The ventral aorta (see Figure 2) ends blindly in front in an anchor- 

 shaped enlargement formed by the bifurcation of the median trunk and 

 the separation of the two resulting vessels, — the anterior innominate 

 arteries, — which curve outwards, upwards, and backwards, quickly 

 splitting into two pairs of vessels the first and second afferent branchial 

 arteries. From the anterior edges of the anterior iunominate arteries, 

 equidistant from the median line, arise two small vessels, which, passing 

 forwards, supply the muscles in the ventral wall of the throat. Other 

 small vascular twigs arise from the ventral aorta as well as the afferent 

 branchial arteries to carry blood to the muscles of the ventral portion of 



