118 ZOOLOGY SECT. 



canal on each side : they are rudimentary neural arches. For the 

 rest of its extent the spinal canal is enclosed only by tough, 

 pigmented connective tissue. 



The cranium also exhibits a very primitive type of structure. 

 Its floor is formed by a basal plate (Fig. 751, ~b.pl.), made by the 

 union of the parachordals and trabeculse, and surrounding pos- 

 teriorly the fore-end of the notochord. Immediately in front of 

 the termination of the notochord is a large aperture, the ba si- 

 cranial fontanelle (b. cr.f.), due to the non-union of the posterior 

 ends of the trabecula? ; through it passes the pituitary pouch, pre- 

 sently to be referred to (Fig. 754), on its way from the olfactory sac 

 to the ventral surface of the notochord. Lateral walls extend 

 upwards from each side of the basal plate, but the roof of the 

 cranium is formed by membrane except at one point, where a 

 narrow transverse bar (cr. r.) extends across between the side-walls 

 and furnishes a rudimentary roof. United with the posterior end 

 of the basal plate are the auditory capsules (au. c), and the side- 

 walls are pierced with apertures for the cerebral nerves (Nv. i.. 

 Xv. o, Nv. 8.). 



So far the skull is thoroughly typical, though in an extremely 

 simple or embryonic condition ; the remaining parts of it differ ;i 

 good deal from the ordinary structure as described in the preceding 

 section, and are in many cases very difficult of interpretation. 



The olfactory capsule (plf. c.) is an unpaired concavo-convex plate 

 which supports the posterior wall of the olfactory sac and is pierced 

 by paired apertures for the olfactory nerves. It is unique in being- 

 united to the cranium by fibrous tissue only. 



Extending outwards and downwards from each side of the basal 

 plate is an inverted arch of cartilage, called the sub-ocular arch 

 (Figs. 750 and 751,s&. oc. a.), from the fact that it affords a support 

 to the eye. From its posterior end a slender styloid process (st. p. } 

 passes directly downwards and is connected at its lower end with a 

 small cornual cartilage (en. c.). In all probability the sub-ocular 

 arch answers to the palato-quadrate or primary upper jaw, the 

 styloid and cornual cartilages to the main part of the hyoid arch. 

 In close relation with the angle of the sub-ocular arch is an up- 

 wardly directed plate, the posterior lateral cartilage (p.lat.c.), which 

 probably answers to the primary lower jaw, or Meckel's cartilage. 

 Connected with the anterior end of the basal plate is the large 

 bilobed posterior dorsal cartilage (p. d. c.) ; it appears to be formed 

 from the united anterior ends of the trabeculae. Below and pro- 

 jecting in front of it is the anterior dorsal cartilage (a. d. c), which 

 is probably homologous with the upper labial cartilage of some 

 Fishes and Amphibia (see below). Also belonging to the series of" 

 labial cartilages are the paired anterior lateral cartilages (a. I. c.) 

 and the great ring-shaped annular cartilage (an.c) which support- 

 the edge of the buccal funnel. 



