316 BULLETIN UNITED STATES GEOLOGICAL SUEVET. [FoI.VI. 



The sub-cylindrical liypo-branchials are one and a half centimetres 

 long, and connected with the posterior elements by quite long and in- 

 tervening piece of cartilage of the same calibre; the smaller cerato- 

 branchials also taper off behind with the same material. 



This arch in the Tetraonidce long remains almost entirely cartilaginous, 

 the hypo-branchial elements alone being composed of bone, and a bird 

 must be of quite an advanced age before he can boast of a complete 

 osseous framework as forming a comjDonent part of his lingual apparatus. 



The third cranial segment, now to be described, is the first of the series 

 in the skull of any of the IsTorth American Tetraonidce in which one of the 

 autogenous elements seems to be generated by osseous extension fi^om 

 continuous parts of the vertebra. The bone in the young and half- grown 

 Grouse is absent, but is eventually replaced in a manner that will be 

 discussed further on. We will ask the reader here, however, need any 

 one doubt or be daunted because an element is missing "? There may 

 still be a greater modification in the ultimate segment ; yet who would 

 question the claim of these cranial arches to their being .defined as verte- 

 brae — distorted and expanded to meet certain required ends, as they are 

 — on such grounds ? Would it be the student who has faithfully carried 

 his observations from the embryo vertebra from the mid-column of the 

 fish, stage after stage, till he has almost unconsciously passed through 

 the multifarious segments described in the adult piscine head, then any 

 of our lizards — or a tortoise, to man himself! Such a one well knows 

 that in every living creature that can vaunt of a vertebral column, that 

 that column's most perfect, complete, and typical segments and arches 

 are found at its centre, and not at its poles. There are ways and modes 

 more difficult in nature of comprehension, as all her students and lovers 

 are well aware, than the fact that the pygostyle at one extreme of the 

 avian skeleton is composed of one or more vertebrae, and the beak, the 

 hsemal spines of two more at the other! 



The neural arch of the vertebra now to be defined is the prosenee- 

 phalic — its hsemal arch the " mandibular, " as its hsemapophysis consti- 

 tutes the lower jaw, termed "mandible" in avian skelejtolo^y. 



The fusing of the centrum of this segment with the basi-sphenoid has 

 already been elucidated -, the rostrum thus formed is gently inclined 

 upwards and forwards, grooved along its entire superior aspect, tapering 

 to a sharp point anteriorly to receive the connate prefrontals in the bony 

 gutter at its distal third. Beneath it displays towards its base the 

 parial facets for the pterygoids and beyond the rounded surface for the 

 palatine articulation. . 



The orUto-spJienoids, as the neurapophyses of this arch, are the ele- 

 ments whose absence has already been hinted at above. 



The author has taken members of this family in sufficient numbers 

 and ages to convince him of the fact that the rotund foramen for the 

 exit of the optic nerve from the brain-case and the smaller aperture for 

 the first pair above, seen in the adult skull, has been slowly formed by 



