THE ELASMOBRANCHII. 135 



more properly, to be considered as the thickened termina- 

 tion of the primitive encephalon, in which the lamina ter- 

 yninalis and the hemispheres are hardly differentiated. 

 The large olfactory lobes are usually prolonged into pe- 

 dicles, which dilate into great ganglionic masses where they 

 come into contact with the olfactory sacs (Fig. 37, A., s). 

 The latter always open upon the nnder- surface of the head. 

 A cleft, which extends from each nasal aperture to tlie 

 margin of the gape, is the remains of the embryonic sepa- 

 ration between the naso-frontal process and the maxillo- 

 palatine process, and represents the naso-palatine passage 

 of the higher Vertehrata. The optic nerves fuse into a com- 

 plete chiasma (Fig. 37, B., ch), as in the higher Vertehrata. 

 In some Sharks, the eye is provided with a third eyelid or 

 nictitating membrane, moved by a single muscle, or by two 

 muscles, an-anged in a manner somewhat similar to that 

 obsei-ved in birds. In both Sharks and Rays, the posterior 

 surface of the sclerotic presents an eminence which articu- 

 lates with the extremity of a cartilaginous stem proceeding 

 from the bottom of the orbit. 



Except in Chimcera, the labyi-inth is completely inclosed 

 in cartilage. In the Rays the anterior and posterior 

 "semicircular" canals are circular, and open by distinct 

 narrow ducts into the vestibular sac. In the other Elas- 

 mobranchii they are arranged in the ordinary way. A 

 passage leading from the vestibular sac to the top of the 

 skull, and opening there by a valvular aperture, represents 

 the canal by which, in the vertebrate embryo, the auditory 

 involution of the integument is, at first, connected with the 

 exterior. 



The testes are oval, and are provided with an epididymis 

 and vas deferens, as in the higher Vertehrata. The vas 

 deferens of each side opens into the dilated part of the 

 ureter. Attached to the ventral fins of the male are pecu- 

 liar appendages, termed claspers. 



The ovaria are rounded, solid organs. There are usually 

 two, but in some cases, as in the Dogfishes and nictitating 

 Sharks, the ovary is single and symmetrical. The oviducts 



