SKULL 



2 I/ 



except where a slender synotic band of cartilage extends 

 between the two periotic capsules. The floor is also incom- 

 plete, a large pituitary fontanelle remaining to indicate the 

 original separation of the trabeculae while transmitting the hypo- 

 physial or pituitary caecum. The notochord traverses the floor 

 of the parachordal portion of the cranium as far as the pituitary 

 fontanelle, and from the sides of the synotic ring the auditory 

 capsules project in the shape of conspicuous lateral prominences. 

 In front the otherwise open end of the cranial cavity is closed 

 by the dorsally-placed and unpaired olfactory capsule, which is 



olf.c 



Lc.i 



pd.c 



i.lat.c 



sb.oc.a 



I.e.* 



L.c.a 



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 ; Ig.c, lingual car- 

 tilage ; m.v.c, median ventral cartilage ; n.a, neural arches ; na.ap, nasal aperture ; 

 n.ch, notochord ; JW 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.) 



perforated behind by two apertures for the olfactory nerves, and 

 has only a fibrous connexion with the cranial walls. Anteriorly 

 to the olfactory capsule the cranial floor is prolonged forwards 

 over the roof of the mouth as a large laterally-expanded plate, 

 formed by the united anterior portions of the trabeculae, and no 

 doubt representing the mesethmoid cartilage of the Dog-Fish. So 

 far the cranium presents no special difficulty, and in its general 

 features may be readily compared with that of an embryonic 

 Elasmobranch. As for the rest of the skull, it is obvious that it 

 has been greatly modified, partly to form and to support the 

 skeletal framework of the remarkable suctorial buccal funnel, and 

 partly to form the singular rasping lingual apparatus. Hence it 

 is always difficult and sometimes impossible to identify with 



