12 DR. J. F. GEMMILL ON [May 12, 



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 cartilag-e. 



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 (PI. II. fig. 10, Meek'). 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 exceedingly reduced in size, the two 

 symphyses lie close together in the tissue of the septum sepa- 

 rating 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 intei'posed the fused remains of the inner arches 

 (fig. 10). 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 hypo- 

 hyal pieces of the outer arches (PI. II. fig. 10, BR), ^o adjacent 

 elements are interposed in the series of branchial cartilages. 

 The only evidence indicating duplicity is to be found in the second 

 copular piece, i. e. that succeeding the glossohyals. This piece is 

 double anteriorly, but it becomes single opposite the articulation 

 with it of the second branchial cartilage. The succeeding copular 

 pieces are single, but they are a little broader than normal, 

 especially in front. 



The notochords remain separate as far back as the twentieth 

 somite. For the arrangement of the neural and haemal arch 

 cartilages in the transition region, see above, page 8. 



There are two sets of brain cavities and masses as far back as 

 the level of the fourth ventricle. The fourth ventricle is single 

 posteriorly, but it bifurcates in front into two canals leading into 

 the separate mid-brains. The posterior part of the medulla and 

 the anterior part of the spinal cord are composite, and show the 

 following characters :— (1) they are much drawn out transversely, 

 and (2) they give origin to small adjacent nerve-roots. These 

 roots ni the medulla are extremely 'rudimentary, and their 

 ultnnate distribution could not be traced, but in the spinal cord 

 the nerve- roots in question are better developed and form a 

 regular series of pairs of nerves coming ofi" from the ventral 

 aspect of the cord and distributed to the somites of the median 



