MUSEUM OF COMPAEATIVE ZOOLOGY. 63 



ral fin, then forward, descending near the head, and, after a backward 

 course of varying extent on the lower surface, unites with the jugular. 

 Or, reversing the direction and starting below, from the jugular, the 

 canal goes out and forward under the pectoral, ascends at the side of 

 the head, then turns out and backward, describing a circuit toward the 

 margins on the top of the fin, and unites with the lateral at the shoul- 

 der. Most frequently it is the case that the pleural and the orbital 

 are connected by orbito-pleural tubules ; exceptionally these tubes meet 

 directly without the intervention of the tubule. No doubt the pleural 

 originated as a branch of the orbital. Besides this pleural canal on 

 the pectoral, there are usually present several others, post-pleurals, 

 from the scapular curve of the lateral toward the hinder part of the 

 fin, which also are not represented in the Galei. Ordinarily the upper 

 pleurals are abundantly supplied with tubules ; sometimes on the lower 

 surface tubules are entirely absent ; and on the Torpedoes the ventral 

 portion of the entire system is obsolete. Branches of tubules are gen- 

 erally in pairs ; a tubule forks to form a pair ; each of this pair forms 

 another pair in similar manner, and so on. This dichotomous branch- 

 ing of the branchlets may be kept up, as in the higher Rays, until on 

 reaching the outer layers of the skin a considerable space is occupied 

 by the mat or rosette formed of the very small tubes and their pore-like 

 openings on each of the tubules. Among the Torpedoes and the Skates 

 the simple unbranched tubule is the common form. 



The origin of the pleurals of the Batoids, or the manner in which the 

 group became possessed of these canals, in addition to those possessed 

 by the Galei generally, is a question of considerable interest. Our only 

 clue to the solution of the problem is to be seen in Chlamydoselachus. 

 If the head and body of this shark were depressed, and the pectorals 

 expanded and applied, so as to produce the skate-like form, the spiracu- 

 lar canal would then extend back along the basal cartilages of these 

 fins, they being attached above the gill openings, thus forming the 

 pleural canal, the union of which with the scapular branch of the lat- 

 eral is only a secondary matter, as shown by the variety in modes of 

 junction, in the Rhinobatidse, the Raiidse, and such genera of Trygoni- 

 dse as Urolophus and allies. On the lower surface the subpleurals 

 would be supplied by the gular and the canal lying between it and the 

 lip, the oral being limited to the part anterior to their point of meet- 

 ing. The fact that the additional canals would be acquired in this way, 

 as a necessary consequence of the change of the form, leads, at the 

 least, to a strong presumption that the Batoidei are indebted for their 



