274 DESCRIPTION OF PLATES. 



FIG. I. Skate, Raja Batis, female, ventral view from a specimen, dissected so as to show the 

 heart, gills, and digestive tract in situ. 



a. The line points to the spot where the conus arteriosus springs from 



the ventricle. This structure lies in front, with the auricle and ductus 

 Cuvieri behind (see Fig. 2, infra). 



b. The line points to the base of the ventral aorta at the spot where it 



springs from the conus, and gives off the two posterior innominate 

 arteries. Each of these vessels divides into three branches the three 

 posterior branchial arteries which run on the outer side of the three 

 posterior (II-IV) branchial arches and supply with venous blood the 

 two gill-laminae, anterior and posterior, borne by each arch. These 

 laminae, together with the fibrous septum which supports them and 

 is continued to the skin separating the so-called gill-pouches inter 

 se, are seen on the right side of the diagram. 



c. The line points to the anterior termination of the ventral aorta where 

 it gives off the right and left anterior innominate arteries. Each of 

 these arteries divides into the two anterior branchial arches which sup- 

 ply the anterior, the single gill-lamina (=opercular gill of Ganoidei, 

 pseudobranch of Teleostei) borne upon the posterior aspect of the 

 hyoidean arch ; the posterior, the two gill-laminae borne by the first 

 branchial arch (I). The fifth branchial arch in the Rays and most 

 Sharks, as in Teleostei and Ganoidei, bears no gill-laminae. The mode 

 in which the branchial arteries arise from the aorta is characteristic 

 of Rays. 



d. The first of the five external gill-slits. The remaining four are seen 



arranged in a curved line behind. Gill-slits uncovered by an oper- 

 cular fold are characteristic of all Sharks and Rays : their completely 

 ventral position, of the latter only. 



e. The aperture into the olfactory pit. This aperture is placed ventrally 



in nearly all Elasmobranchii. A groove leads from the pit to the 

 corresponding angle of the mouth. Such a groove exists in the em- 

 bryoes of all higher Vertebrata. The outer edge of the groove re- 

 presents the fronto-nasal, the inner edge the praemaxillary, processes 

 seen in the embryoes of Vertebrata which possess praemaxillary 

 and maxillary bones. If the roof of the olfactory pouch is examined 

 in a specimen it will be seen to possess two series of transverse folds. 



y. The line rests upon the upper jaw, which is cartilaginous, and repre- 

 sents, as in all Elasmobranchii, a palato-pterygo-quadrate cartilage. 

 The transverse slit of the mouth and the under jaw ( = distal end of 

 Meckel's cartilage) are seen with the rows of diamond-shaped teeth 

 set edge to edge. The retention of a ventrally placed mouth is 

 characteristic of the Elasmobranchii. 



g. Jelly tubes or sensory ampullae. Only a certain number of these 



