102 SUB-CLASS MAKSIPOBKANCHII (CYCLOSTOMATA). 



nerve passes out through the auditory capsules. Tlie vagus 

 nerve passes out behind the skull, between it and the first dorsal 

 piece of the vertebral column (Fig. 50, 6). 



In its general features the chondro-cranium of Marsipohranchii resembles 

 that of other fishes, but is more largely supplemented by membranous 

 structures, nearly the whole of the roof, the entire occipital region and 

 the basicranial fontanelle being membranous. From what is known of 

 the development of the lamprey's skull, it would appear that the basilar 

 plate is formed by two parachordals between which the notochord hes 

 (Fig. 52), that these become continuous above and below the notochord 

 (below only in Myxine), and that to their outer sides the auditory capsules 

 become attached. In front of the parachordals and continuous with them 

 are the trabeculae cranii which always remain separate posteriorly, leaving 

 the basicranial fontanelle, but imite in front to form in Petromyzon (Fig. 52) 

 the hard palate, the ethmoid and the posterior dorsal plate (ethmovomerine 

 plate), and in Myxine (Fig. 53, 13) a small median piece in front of the 

 large basicranial fontanelle. In front of the latter there is in Myxinoids a 

 pair of cartilaginous horns (Fig. 53, 17) which may be regarded as the 

 homologues of the posterior lateral cartilages of the lamprey. Moreover 

 in Myxinoids there are two median cartilages, called intertrabecular car- 

 tilages not present as separate structures in Petromyzon ; one of these — 

 the posterior intertrabecular — is a spoon-shaped cartilage lying in the basi- 

 cranial fontanelle and underlying the naso-palatine canal ; the other, 

 the anterior intertrabecular, extends in front of this and lies beneath the 

 nasal canal. In all Marsipobranchs there is a ventre -lateral process of 

 the hinder part of the trabecular region (anterior lateral process) which 

 meets and fuses with a corresponding process of the anterior end of the 

 basilar plate or auditory cartilage (posterior lateral process, said to be 

 comparable to the pedicle of the Amphibian suspensoriimi. Fig. 53, 7) ; 

 thus forming the so-called subocular arcade. The subocular arcade closely 

 resembles the same structure in the Amphibian skull, but against the 

 comparison of the two is the fact that the ventral division of the fifth 

 nerve passes dorsal to the arcade in Amphibians, whereas in Marsipo- 

 hranchii it passes through the fenestra (Fig. 51, 18). In Myxinoids the 

 hinder part of the subocular arcade contains two fenestrae (Fig. 53,6, 21), 

 which are not present in lampreys. The supposed hyoid arch arises in 

 all Marsipobranchs from the hind end of the subocular arcade (Fig. 50, 18 ; 

 Fig. 53, 23). In lampreys it ends in the expansion of the cornual cartil- 

 age (Fig. 50, 19) and is not connected with the lingual {15) which is the 

 supposed median element of the hyoid arch. In Myxinoids it joins the 

 great lingual cartilage (Fig. 53, 18) which consists of several parts. In 

 the same group the hinder part of the subocular arcade also gives off 

 close to the point of origin of the hyoid a bar of cartilage which passes 

 ventralwards just within the hypoblastic epithelium to join the subocular 

 arch lower down (Fig. 53, 24:). This structure is not represented in 

 lampreys and is supposed to be the first branchial arch. The velum in 

 Myxine is supported by some pieces of cartilage which are in connection 

 with the upper end of this supposed branchial arch (Fig. 53, 2, 3). 

 ' In Myxinoids the brain lies entirely above the cartilaginous skull, which 

 is a mere floor, the side walls and roof being entirely formed of membrane. 

 Moreover in Myxinoids the angle of the subocular arcade is posterior (Fig. 



