OSTEOLOGY, ETC., OP SYNGNATHUS PECKIANUS. 645 



first stage in the phylogenetic history of the group. The 

 second stage of union is also well marked, and it is not until 

 comparatively late in life that the commencement of the third 

 stage, represented by the beginning of the latei-al upward 

 growth, makes it appearance ; and it may be explained by the 

 function the trabeculae have in the adult of supporting the 

 facial region, and protecting the olfactory sense organs. This 

 specialization is only a degree greater than that found, for 

 instance, in the second postoral arch, where a longitudinal 

 division occurs, the posterior rod forming the hyoid, for the 

 support of the tongue and aiding in the respiratory process, 

 and the anterior one the syraplectic, uniting the mandible to 

 the skull, and serving for its support. 



The relation, too, of these trabeculae to the pituitary fossa is 

 also importfint. They articulate with the vertebral region of 

 the skull immediately behind that structure, and, in fact, have 

 the same relation to it as the other arches have to their clefts. 

 By being bent up, and by the atrophy of the pituitary cleft, 

 they eventually lose their original relations, and enclose them 

 instead of being their posterior boundaries. 



The following table, which is a modification of Marshall's 

 table,^ itself a modification of one by Balfour,^ will show at a 

 glance the relations which I consider probable as existing be- 

 tween the various segments. As in Marshall's table, clefts, &c., 

 have been allowed for the Notidani. 



' A. Milnes Marshall, " The Morphology of the Vertebrate Olfactory Or- 

 gan," ' Quart. Journ. Micr. Sci.,' vol. xix, 1879. 



2 F. M. Balfour, " On the Development of Elasmobranch Fishes," ' Journ. 

 of Anat. and Phys.,' vol. xi. See also 'Monogr. of Elasmobranchii,' 



VOL. XXIII. NEW SER. V V 



