1903.] MONSTKOSITIES IN FISHES. 11 



room for greater development on the part of the inner or adjacent 

 elements in the twin heads (PL I. fig. 5). The two converging- 

 pairs of trabecnlfe are separate along their whole length, and each 

 unites posteriorly with corresponding parachordals. The two 

 pairs of parachordals are separate in front, but posteriorly the 

 adjacent elements in each pair unite, so that a single composite 

 basilar plate of cartilage containing two notochords underlies the 

 lower half of the medulla oblongata. The inner or adjacent 

 palato-quadrates converge posteriorly and coalesce. The united 

 part articulates (1) below, with a small bifid cartilage representing 

 fused adjacent Meckelian bars ; and (2) higher up, with a small 

 cartilage representing fused adjacent hyomandibulars. The inner 

 or adjacent supraorbital bars converge posteriorly, unite with one 

 another, and end by becoming continuous with the roof of a small 

 box of cartilage which is wedged into the apex of the angle 

 between the twin heads and represents fused adjacent periotic 

 capsules. This structure will be described later on, but it may 

 be mentioned here that its roof is continuous posteriorly with a 

 vault of cai-tilage which connects the two outer (noi'mal) periotic 

 capsules over the cerebella. In this way double sets of anterior 

 and middle fontanelles are left over the cerebral lobes and mid- 

 brains respectively of the twin heads (PI. I. fig. 6). 



The inner or adjacent hyomandibulars are extremely rudi- 

 mentary, and are fused together to form a small bifid piece which 

 articulates (1) anteriorly, with the fused adjacent palato-quadrates, 

 (2) posteriorly, with the fused adjacent periotic capsules, and (3) 

 inferiorly, with a rudiment of the fused adjacent hyoid bars. The 

 small artery for the supply of the innei- or adjacent pseudobranchs 

 passes up through the notch at the anterior end of this cartilage. 

 The inner or adjacent auditory capsules are extremely rudi- 

 mentary, being completely txnited, compressed from side to side, 

 and wedged into the position above indicated. They contain a 

 single distorted labyrinth, and are entered by small auditory 

 nerves on either side, which are distributed symmetrically over 

 the labyrinth. This composite auditory capsule is connected 

 anteriorly with the fused adjacent supraorbital bars, and ventrally 

 with the adjacent trabecular and parachordal cartilages on the 

 inner sides of the two pituitary spaces. Behind it a small tri- 

 angular opening is left, bounded on either side by the converging 

 parachordals. These unite posteriorly, but leave a narrow 

 foramen between them for the exit of a small nerve, which 

 represents a reduced adjacent pair of vagus and glosso- pharyngeal 

 nerves (PI. I. fig. 5). It will be seen from what precedes that 

 there are five fontanelles, one over each pair of cerebral lobes, one 

 over each mid-brain, and one over the composite medulla 

 oblongata (PI. I. fig. 6). 



There are two mouth-openings placed side by side, one under 

 each head, and separated from one another by a thick dorso- 

 ventral septum. This septum contains (1) remains of the adjacent 

 mandibular and hyoid cartilages, (2) much confused muscular 



