100 



SUB-GLASS MAKSIPOBRAXCHII (CYCLOSTOMATA). 



O..A<*^X*^44 



C^^A*^JU.—^ 



compared to the hyoid arch of the higher forms. In the 

 Petromyzontidae the following additional cartilages are present. 

 Attached to the anterior end of the base of the skull is a large 

 median plate of cartilage — the posterior dorsal cartilage 

 (Fig. 50, 10, and Fig. 51, 11). Immediately in front of this, 



and overlapped by it, is the anterior 

 dorsal cartilage (Fig. 50, 11, and Fig. 

 51, IJ/)- Just below the latter there 

 is an annular cartilage (Fig. 50, 12, 

 and Fig. 51, 15), which lies within the 

 lips, and from which there projects 

 back on each side the so-called styli- 

 form cartilages (Fig. 50, 14)- The 

 anterior lateral cartilages are paired 

 rods (Figs. 50 and 51, 13) in relation 

 with the anterior dorsal cartilage, and 

 the posterior lateral (Fig. 50, 16, and 

 Fig. 51, 10) are similarly in relation 

 with the posterior dorsal. 



Finally, in the tongue in the floor 



of the mouth there is a median 



petromyzon marinus fafter""w! Hngual Cartilage (Fig. 50, 15, and Fig. 



K. Parker). 1 posterior dorsal pri ^^\ j -r^pTitrnl fn tTii« tViA en 



cartilage (ethmovomerine platp), o 1. , 1 / ) , A;l\U. venuai tO l-nis tne SO- 



Sc'uiaS%assil,T be^indTnto called median ventral cartilage (Fig. 



;VSioVS^n°™?raKre; 51^ ^^ ; ^ot shown in Fig. 50). The 



Lfa'defl*Sid^'o?e'sf'^6 lingual Cartilage is also present in 



Its ^rtfoVinvesS'ma'J Myxinoids, in which it is Very largely 



(formed by union of parachorial /-l^^-iTplrii-x^rl 



cartilages), showing contained ^leveiupeu. 



notochord ; 9 basi-cranial fonta- 



nelle, or space between the t, ■ uiij.ii.ii j. • i i 



unfused posterior portion of ^^ ^^ probable that the posterior dorsal 



trabeculae ; 10 cartilage formed cartilage is derived from the fused anterior 

 by fusion of middle portion of j r j.i. j. i, i j -j. i i 



trabeculae (so-called hard palate end of the trabeculae and it has been 



of J. Miiller) ; it lies between suggested that the posterior lateral cartil- 

 mouth and posterior nasal pouch, j.i, i, i r -nr i i> ^-i 



and corresponds to the posterior ages are the homologues of Meckel s cartil 



intertrabecula of myxinoids. age. The anterior dorsal, and the anterior 



laterals, and the annular cartilages are 

 generally regarded as labials, while the lingual cartilage has been compared 

 to the basi-hyal ; it is connected with the styloid process (supposed hyoid) 

 in Myxinoids (Fig. 53). 



*io. 52.— Ventral view of skull of 



The foramen for the optic nerve is in the side wall of the skull 

 above the subocular arch (Fig. 50, 8), that for the fifth nerve 

 just in front of the auditory capsule (Fig. 50, 7), while the seventh 



