144 PROFESSOR MARSHALL. 



Iii the case of the lampreys, then, I hold that we have no reliable 

 evidence of the third or fourth nerves being in any way abnormal in 

 their distribution to the eye-muscles ; while, as regards the sixth 

 nerve, although no distinct root of origin has yet been seen, I hold, 

 Avith Stannius, that a much more careful and searching investigation 

 must be made for it before any statement as to its absence can be 

 accepted. 



3. Ganoidei. — In the majority of Ganoids the nerves of the eye- 

 muscles have the normal arrangement, and are completely indepen- 

 dent of the fifth, except where the third unites with the ophthalmic 

 branch of the fifth at the ciliary ganglion. Only one exception is 

 known. 



In Lejndosteus according to J. Midler, 1 the arrangement is abnormal, 

 the third and fourth nerves entering the orbit closely united with the 

 ophthalmic division of the fifth, of which they appear as branches. 

 The sixth nerve is described and figured as accompanying the main 

 trunk of the fifth, but distinct from it. 



Stieda, in his essay 2 before referred to, quotes Midler's account, but 

 does so incorrectly, making Midler say that there is a distinct fourth 

 nerve, but that the third and sixth are replaced by the branches of 

 the fifth ; whereas Midler really says that the sixth is a distinct nerve, 

 and that the third and fourth are, not " replaced by branches of the 

 fifth," but contained in the ophthalmic nerve. 



Stannius, 3 referring to Midler's account, observes that it is pro- 

 bably merely another instance of juxtaposition of originally distinct 

 nerves. 



Concerning this alleged exception, we notice in the first case that 

 it rests on a solitary description, which has not yet been confirmed, and 

 that confirmation is needed is evident from the figures referred to. 

 Midler gives two figures of the cranial nerves of Lepidosteus, which 

 do not agree in all points ; indeed, the points of difference are so 

 marked that the two figures are by no means easy to reconcile 

 with one another. Midler's figure 3 appears to me to present nothing 

 exceptional, except that the third and fourth nerves enter the orbit as 

 one trunk, and that the fourth nerve at the point where it crosses the 



1 J. Miiller, Ueber den Bm und die Grenzen der Ganoiden, 1846, p. 97, and plate iv. figs. 2 

 and 3. 



2 Stieda, loc. cit., p. 174. 



3 Stannius, Das peripherische NerVensystcm der Fiscke, p. 19, 



