336 DR. A. MILNES MARSHALL. 



here only notice briefly one or two points of importance. 

 On the yiew here put forward the trabeculce cranii, which 

 lie at the base of the brain, run parallel to the longitu- 

 dinal axis of the head, and at right angles to the segmental 

 structures, such as the third and olfactory nerves and 

 the corresponding clefts, must be regarded as axial struc- 

 tures, and not as arches, whether neural or haemal.^ 



Again, my investigations appear to leave no room for 

 doubt that the maxillary arch, the rudiment of the upper 

 jaw, is as fully entitled to rank as a distinct visceral arch as 

 the mandibular, hyoid, or branchial arches. Figs. 3 — 6 

 appear to me to afford conclusive evidence on this point. 



The morphological nature of the labial cartilages has been 

 matter of much dispute ; if the determination of the olfac- 

 tory organ as a gill cleft be accepted, those at least of the 

 labial cartilages which are grouped round the external aper- 

 ture of the olfactory organ, and very possibly those* also in 

 connection Avith the gape of the mouth, would appear to be 

 homologues of the extra-branchial cartilages, a suggestion in 

 which I find I have been anticipated by Professor Parker.- 



I cannot refrain here from referring to the remarkable 

 manner in which the views here put forward agree with the 

 results arrived at by Professor Parker, and embodied in his 

 latest paper .^ He there expresses himself " satisfied that, 

 in spite of the doubling up of the basis cranii, at the time 

 of its greatest flexure, there are rudiments of three prseoral 

 arches, related to ttvo prceoral clefts, the lachrymal and the 

 nasal." " Thus we get four pre-auditory, and eight post- 

 auditory clefts, with their nerves ; if we add the twelfth 

 (hypoglossal), of the ' Amniota,' we have obtained signs and 

 proofs of thirteen cranial (segmental) nerves, all of these, ex- 

 cept the last, forking over visceral clefts, and hedged in all 

 hut the last by visceral bars. The first of the bars is in 

 front of the first or tiasal cleft, the last, or thirteenth, is 

 the hinder bar of the lamprey's branchial basket work." 

 The italics in the above quotation are mine. Though I see 

 no reason for regarding the hypoglossal as a segmental 

 cranial nerve, this extract from Professor Parker's work 

 shows that the study of the skeletal elements of the head 



1 Cf. Parker, " On the Development of the Skull and its Nerves in the 

 Green Turtle," ' Proc. Royal Soc.,' 1879. 



2 'Trans. Zool. Soc.,' 1876, "On the Structure and Development of the 

 Skull in Sharks and Skates," pp. 212 and 224. 



3 " On the Development of the Skull and its Nerves in the Green Turtle." 

 'Proc. Royal Soc' 1879. This vras read before the Royal Society on the 

 same evening as the abstract of the present paper. 



