100 



SUB-CLASS MARSIPOBRANCHII (CYCLOSTOMATA). 



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, 14)- 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 



i 1 - T , fT11Q l oartilarm /TTifr ^O 7o nnH T^icr 

 lingual Cartilage (JJ Ig. OU, lO, and 



KI 1?\ QTir l TTanfral +n +Vii tVif* <an 

 ^ l > U ), ana Ventral 



no ]]^ rnorlian Tr^ntral r>artilao-^ fTTio- 



called median ventral cartilage ^ig. 

 61, ^; not shown in Fig. 50). The 

 lingual cartilage is also present in 

 Myxinoids, in which it is very largely 



8 



FIG. 52. Ventral view of skull of 

 Petromyzon marinus (after W, 

 K. Parker). 1 posterior dorsal 

 cartilage (ethmoyomerine plate), 

 formed by union of cornua 



trabecuiae, passing behind into 



(formed by union of parachorial 

 cartilages), showing contained 

 notochord ; 9 basi-cranial fonta- 



Snfu e sed r po 8 s P t a edor ^pSn *? It is probable that the posterior dorsal 

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

 && t-SSVa^aVg end of the trabeoula* and it has been 

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

 SS*SM?to r firSSS& 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). 



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 



