NERVES OF THE DOGFISH 351 



that contains a well-developed sense-organ in its posterior end. 

 The tubular organ in question lies anteroventrally to the man- 

 dibular canal, opening to the exterior at its own anteroventral 

 end. It is about 0.7 mm. in length. It is apparently a modified 

 ampulla of Lorenzini. From this tubular organ the anterior 

 branch of the ramus mandibulars externus continues in an 

 anteroventral direction, in close proximity to the anterior sen- 

 sory division of the ramus mandibularis V, and finally comes to 

 a series of tubular organs ventral to the lateral border of Meckel's 

 cartilage, between the latter and the skin. These tubular organs 

 all open upon the mucous membrane, or near the line of junction 

 of the mucous membrane with the epidermis at the lateral angle 

 of the mouth. In the single specimen in which they were care- 

 fully examined they are eleven in number, and are evidently the 

 rudiments of a group of ampullae of Lorenzini. All these 

 tubular organs are innervated by the nerve branch mentioned 

 above. 



The main part of the posterior division of the ramus mandibu- 

 laris externus supplies the hyoidean group of ampullae. It 

 passes posteroventrally from the point of division as a stout 

 nerve that suddenly breaks up into its terminal twigs for the 

 rosette of hyoidean ampullae. From the base of the posterior 

 division of the ramus mandibularis externus there runs anteriorly 

 along the lateroventral border of the adductor mandibulae 

 muscle a small nerve whose destination is a series of pit-organs 

 in the jugular region, lying mostly ventral to the hyomandibular 

 canal. There are about twenty-five of these pit-organs. Farther 

 dorsally is another series of pit-organs innervated by small 

 twigs from the main hyomandibular trunk before the branching 

 of the latter. There are about eight pit-organs in this second 

 group. The two groups constitute a single series almost in the 

 shape of a semicircle, extending from the spiracle nearly to the 

 midventral line. 



Hawkes finds in Chlamydoselachus three branches of the ramus 

 mandibularis externus which seem to correspond in a general way 

 to the dorsal and ventral branches of the anterior division and the 

 entire posterior division in Squalus. She notices several branches 



