1903.] MONSTROSITIES IN FISHES. 13 



muscular mass which lies ventral to the notochords (compare 

 page 9). In this monstrosity, as in the one previously described, 

 the anterior part of the spinal cord, though it lies nearer to the 

 place of union of twin bodies, shows greater structural duplicity 

 than does the medulla oblongata. 



All the outer cranial nerves belonging to the twin heads are 

 normal and need no further mention. Of the inner or adjacent 

 nerves, the 1st, 2nd, 3rd, and 4th are normal; the 5th and 

 its ganglia are reduced in size; the 7th and 8th with their 

 ganglia are very rudimentary ; while only a remnant of the 

 adjacent glosso-pharyngeal and vagus i-emains. I was unable to 

 follow out the 6th pair, but the presence of well-developed 

 external rectus muscles makes it probable that these nerves were 

 present. As has already been mentioned, the inner or adjacent 

 auditory capsules are much reduced in size, their auditory sacs 

 being completely united and forming a single labyrinth, sym- 

 metrical in shape, compressed from side to side, and receiving the 

 two adjacent auditory nerves. The arrangement of the sensory 

 epithelium inside the various parts of this labyrinth is also 

 bilaterally symmetrical. A reconstruction drawing of this 

 labyrinth is given in PI. IV. fig. 27. Saccule, utricle, and 

 anterior and posterior semicircular canals are all represented, but 

 there is no trace of a horizontal semicircular canal. 



The heart is normal and gives origin to a single ventral aorta, 

 which for a short distance upwards has a double cavity, owing to 

 the presence of a median antero-posterior septum, which, how- 

 ever, disappears further forwards. The gill-arteries on either 

 side are normal, but in addition to them the ventral aorta gives 

 rise to several small irregular branches which ramify in spongy 

 tissue surrounding the ventral ends of the branchial cartilages, 

 and which may be taken to represent a very rudimentary set of 

 adjacent gill-arteries. But the most stiiking feature of the 

 ventral aorta is that, instead of ending in the first gill-arteries, it 

 is continued forwards and arches dorsally in the tissue of the 

 septum between the two mouth-openings. Passing through the 

 space between the adjacent giossohyals and the succeeding copular 

 piece, it comes to lie behind the small cartilage which represents 

 adjacent ceratohyals. Then, reachmg the base of the skull, it 

 bends backwards, and divides into two equal branches which join 

 the upper aortic roots on either side (PL II. fig. 13). In the first 

 part of its course, this continuation of the ventral aorta gives ofi" 

 (1) two inner or adjacent carotid arteries, which, after running 

 forwards and outwards, pass through their corresponding pituitary 

 spaces ; and (2) two arteries for the supply of the inner or adjacent 

 pseudobranchs. These arteries run at first forwards and dorsal- 

 wards behind the adjacent giossohyals ; then curving forwards 

 they unite in front of the adjacent ceratohyals ; then, separating 

 again, they pass between the adjacent palato-quadrates and the 

 hyomandibulars, and are distributed to their correspondmg (i. e. 

 inner) pseudobranchs. On either side the first aortic root gives 



