122 



ZOOLOGY 



SECT. 



of the fin. For the rest of its extent the spinal canal is enclosed 

 only by tough, pigmented connective-tissue. Slender rods of 

 cartilage support the median fins. 



CL.d.C 



+ nc/t 



p.La.1.0 



cri.c 



FiQ. 805. Petrpmyzon marinus. Dorsal (A), ventral (B), and sectional ((7) views of skull. 

 The cartilaginous parts are dotted, a. d. c. anterior dorsal cartilage ; an. c. annular cartilage ; 

 au. c. auditory capsule ; b. cr. f. basicranial fontanelle ; b. pi. basal plate ; era. c. cornual 

 cartilage ; cr. r. cranial roof ; n. a. neural arch ; na. ap. nasal aperture ; nch. notochord ; 

 Nv. 1, olfactory nerve ; Nv. 2, 5, and 8, foramina for the optic, trigeminal, and auditory 

 nerves ; Nv. 5', fifth nerve ; olf. c. olfactory capsule ; p. d. c. posterior dorsal cartilage ; 

 p. lot. c. posterior lateral cartilage ; s6. oc. a. sub-ocular arch ; st. p. styloid process. (After 

 W. K. Parker.) 



The cranium also exhibits a very primitive type of structure. 

 Its floor is formed by a basal plate (Fig. 805, 6. pi.), made by the 



