OSTEOLOGY, ETC., OF SYNGNATHUS PECKIANUS. 641 



apposition of the former to the hyomandibular, the abortion of 

 the geniohyoid, the absence of the intermaxillaries and the 

 preopereula^ are the most important points. 



During the past few years, chiefly through the investigations 

 of Parker, Balfour, Gegenbaur, and Marshall, much light has 

 been thrown upon the segmentation of the skull and the rela- 

 tions of the visceral arches. Tliere has been an increasing 

 tendency to refer the various subcranial arcades to the category 

 of visceral arches, and to increase the number of segments 

 which have become coalesced to form the cranium. 



It is now well recognised that all the visceral arches behind 

 the mouth, including the mandibular and hyoid arches as such, 

 are merely the modification of a series of cartilaginous bars situ- 

 ated in the walls of the pharynx, and originally supporting 

 branchiae. The posterior visceral arches, to the number of five, 

 increasing to six or seven in the Notidani, and diminishing to 

 four in Syngnathus, still retain this original function, but it 

 is not so with the two anterior postoval arches ; these have be- 

 come modified to subserve other purposes, and have lost to a 

 large extent their original structure and appearance. The distri- 

 bution of cranial nerves to these, however, resembles that of the 

 branchial arches, the trigeminal being referable to the mandibu- 

 lar, and the facial and the'auditory (which from embryological 

 facts may be considered as one) to the hyoid arcade and the adja- 

 cent structures. The visceral clefts point to the same conclu- 

 sion. Each visceral arch bounds posteriorly a visceral cleft ; 

 so for the hyoid arch we have the spiracular cleft, and for the 

 mandibular the mouth, since Dohrn's late researches^ show 

 that this is formed by the coalescence of two hypoblastic out- 

 growths, the median opening only forming later. From the rela- 

 tions of the head cavities one avou Id deduce the same conclusions. 

 Balfour, who has worked them out very thoroughly in the 

 Elasmobranchs, thus speaks of them :^ — "As the rudiments of 



^ A. Dobrn, " Studien zur Urgescliichte des Wirbelthierkorpers," 'Mith. a 

 d. Zool. Station zu Neaple,' 13d. iii, 1882. 



- F. M. Balfour, loc. cit., p. 558. See also ' Mouograph of Elasmobranch. 

 Fislies.' 



