| OCR Text |
Show 1903.] MONSTROSITIES 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 trabecular 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 cartilage which connects the two outer (normal) periotic capsules over the cerebella. In this way double sets of anterior and middle fontanelles are left over the cerebral lobes and midbrains respectively of the twin heads (PL I. fig. 6). The inner or adjacent hyomandibulars are extremely rudimentary, 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 arteiy for the supply of the inner or adjacent pseudobranchs passes up through the notch at the anterior end of this cartilage. The inner or adjacent auditory capsules are extremely rudimentary, being completely united, 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 triangular 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 (PL 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 (PL 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 |
