Having now considered the general structure of a primitive cartilaginous type of skull, and the nature, disposition, and terminology of the various membrane- and cartilage-bones which may be added to, or more or less completely replace the former, reference will now be made to the more important features in the structure of the skull in the Cyclostomata and the Fishes. In the Cyclostomata the skull presents a remarkable combination of characters, in some of which it is more primitive than in any other Craniates, while in others it has evidently attained a very high degree of specialisation on lines peculiar to the group, but differing in the two subdivisions. {217}[Illustration: FIG. 124.—Skull, with branchial basket and anterior part of the vertebral column, of _Petromyzon marinus_. _a.d.c_, Anterior dorsal cartilage; _a.lat.c_, anterior lateral cartilage; _an.c_, annular cartilage; _au.c_, auditory capsule; _br.b.1-9_, vertical bars of the branchial basket; _br.cl.1-7_, external branchial clefts; _cn.c_, cornual cartilage; _cr.r_, cranial roof; _l.c.1-4_, longitudinal bars of branchial basket; _lg.c_, lingual cartilage; _m.v.c_, median ventral cartilage; _n.a_, neural arches; _na.ap_, nasal aperture; _n.ch_, notochord; _Nv^2_, foramen for optic nerve; _olf.c_, olfactory capsule; _pc.c_, cartilage surrounding pericardial cavity; _p.d.c_, posterior dorsal cartilage; _p.lat.c_, posterior lateral cartilage; _sb.oc.a_, subocular arch; _st.p_, styloid process; _sty.c_, styliform cartilage; _t_, teeth. (From Parker and Haswell, after W. K. Parker.)]