16 DOUBLE MONSTROSITY— STRUCTURE, CLASS II. 



twin heads, but the union is again of the simple lateral type. The two pairs of trabeculae converge 

 posteriorly, but are separate along their whole length, and unite with corresponding parachordals. The 

 latter 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 posterior 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, wedged into the apex of the angle 

 between the twin heads, and representing fused adjacent periotic capsules. This structure will be 

 described later, 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 mid-brains 

 respectively of the twin heads. 



The inner or adjacent auditory capsular cartilages 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 receive 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. 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. 



Mouth. There are two mouth-openings placed side by side, 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 tissue, and (3) two arteries which will be afterwards 

 described, one being a continuation of the ventral aorta, and the other a small artery for the supply 

 of the inner or adjacent pseudobranchs. The two mouth-openings lead into separate buccal cavities, 

 but the oesophagus is single, the septum above mentioned ending opposite the second branchial 

 cartilage. 



Visceral Arch Skeleton. 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) inferiority, 

 with a rudiment of the fused adjacent hyoid bars. The small artery for the supply of the inner or 

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



The mandibular apparatus may be described as consisting of a composite arcade underlying the 

 two mouth-openings. The outer portions of this arcade are formed by normal (outer) Meckelian 

 bars (i.e. right bar of right twin head and left bar of left head), while a small mesial portion of the 

 arch is formed by rudimentary adjacent Meckelian bars. These latter bars are united at their 

 proximal ends, and there articulate with the fused adjacent palato-quadrates. Distally, each of 

 the inner bars unites in a symphysis with its corresponding outer Meckelian bar. As the inner 

 bars are extremely reduced in size, the two symphyses lie close together in the tissue of the 

 septum separating the two mouth-openings. The hyoid apparatus may also be described as forming 

 a composite arcade, the main part of which consists of the outer arches of the twin heads, while in 

 the middle are interposed the fused remains of the inner arches. 



These remains consist of (1) a single twisted piece of cartilage articulating, without the 

 intervention of an interhyal, with the fused adjacent hyomandibulars, and representing cerato- 

 hyals ; and (2) two incompletely separated hypohyals, articulating below with two glossohyals which 

 are also incompletely separated. Connected with the outer sides of these glossohyals are the 



