126 



ZOOLOGY 



SECT. 



of the basal plate are the auditory rapsules (ait. r.), and the side- 

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

 Nv. 5, Nv. 8). 



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

 simple or embryonic condition : the remaining ])arts 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 (jlfactory 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 svh-ocular arch 

 (Figs. 794 and 795, sb. oc. a.) from the fact that it affords a support 



Mil hr.bs hrh.-.' I c^ 



pdc nn.uf 



■it>ailiirririiYit^"'" 



sb.oc.a 



hr.cl.y 



Fk;. 704.— Petromyzon marinus. skull, with Ijnaivliial basket and anterior part of verte- 

 bral cohiniii. The cartil.igincms parts are dotted, a. d. c. anterior dor.sal cartilage ; a. lat. c. 

 anterior lateral cartilage ; an. c. annular cartilage ; cm. c. audit'jry capsiile ; ///■. b. 1 — 7, verti- 

 cal bars of branchial ba.sket ; hr. cl. 1 — 7, external branchial clefts ; ni. c. cornual cartilage ; 

 rr. r. cranial roof ; I. c. 1 — 4, longitudinal bar.s of branchial basket ; lij. c. lingual cartilage ; 

 m. V. c. median ventral cartilage; n.a, neural arch ; nu. ap. nasal aperture ; nek. notochord ; 

 Hi-, i', foramen for optic nerve ; olf. c. olfactory capsule ; ;)c. c. pericardial cartilage ; p. iL c. 

 posterior dorsal cartilage ; p. lat. c. posterior lateral cartilage ; .y/i. oc. o. subocular arch ; st. "p. 

 styloid process ; stii. c. styliform cartilage ; t. teeth. (After W. K. Parker.) 



to the eye. From its posterior end a slender sti/loid 'process {st. j>.) 

 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 

 upwardly directed plate, the posterior lateral carlihnjc {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 trabecuhf. 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 arc the paired anterior lateral cartilages {a. I. c.) 



