xni PHYLUM CHORDATA 123 



union of the parachordals and trabeculse, and surrounding pos- 

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

 the termination of the notochoKl is a large aperture, the basi-cranial 

 fontanelle (b. cr. /.), due to the non-union of the posterior ends of 

 the trabeculse ; through it passes the pituitary pouch, presently 

 to be referred to (Fig. 808), 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. 2, Nv. 5, Nv. 8.) 



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

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

 good deal from the ordinary structure as described in the preceding 

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



The olfactory capsule (olf. 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. 804 and 805, sb. 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.). Perhaps 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 upwardly 

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



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 Amphibians (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 supports 

 the edge of the buccal funnel. 



The " tongue " is supported by a long unpaired lingual cartilage 

 (Fig. 804, Ig. c.), which may answer to the united Meckel's cartilages 

 or ventral portion of the mandibular arch of other Crania*ta (see 

 p. 75) ; it is tipped in front by a small median and a pair of still 

 smaller lateral cartilages. Below it is a slender T-shaped median 

 ventral cartilage (m. v. c.), which may possibly be the median ventral 

 element of the mandibular arch. Lastly, attached to each side of 



