4 ZOOTOMY. 



3. The fin-rays (Figs. 7 and 8,/r), delicate cartilaginous 

 filaments, supporting the dorsal and caudal fins (§§ 29, 30). 

 They are connected ventrally with the fibrous tissue cover- 

 ing in the neural canal, and lie parallel to one another and 

 inchned backwards. 



4. The brain-case, or hindmost division of the skull. 

 Posteriorly it consists of a basal or parachordal plate 

 (Figs. I and 2, b.p, b.p'), surrounding the anterior end of 

 the notochord, and divided by the latter into dorsal ip.p) 

 and ventral (b.p') portions. The ventral portion forms a 

 continuous plate beneath the anterior end of the notochord 

 (Figs. 2 and 4, b.p') ; the dorsal portion consists of two 



Fig. 2. — Petromyzon marinus. The brain case, from above, the 

 subocular arches supposed to be removed (nat. size : from a larger 

 specimen than Fig. i). 



au, auditory capsules : b.p, dorsal, and b.p', ventral portion of 

 basal or parachordal plate : h.p, hard palate : na.p, naso-palatine 

 canal : oc, occipital arch : tr, trabecula : w, side wall : II, optic fora- 

 men . V, trigeminal foramen. 



longitudinal plates, which do not unite over the notochord. 

 Anteriorly the basal plate is continued forward into two 

 flattened plates, the trabeculae (Fig. 2, tr), which unite 

 with one another in front, forming the hard palate {h.p), 

 and inclosing an oval space, the naso-palatine canal 

 {na.p). Laterally, the trabecule and hard palate send up 

 plates of cartilage (w), which afford side walls to the brain- 



