No. 2.1 SHUFELDT ON NORTH AMERICAN TETRAONID^. 313 



depression a group of usually four foramina — two external opening into 

 the otocrane, one into the cranial cavity, and one leading through the 

 basi-sphenoid to the base of the ' sella turcica ' at the carotid openings ; 

 they transmit principally the eighth nerve and the internal jugular and 

 branches. 



In some of the very old indivkluals of the Tetraoninm quite a striking 

 characteristic presents itself in the capaciousness of the opening to the 

 otocrane, produced by a thin, wing-like expansion, recurved forwards, 

 formed by the out-growing and union of the centrum of the second 

 vertebra and the diapophysis of the first. This feature is not particu- 

 larly noticeable in the Odontophorincc, nor in Lagopus, Ctipidonia, and 

 Bonasa, still less so in the Sharp-tailed Grouse, among the Tetraonince, 

 but quite marked in old males, especially in Tetrao and Centrocercus 

 (Figs. 52, 74, 88, aud 89). Ko very decided differences exist among the 

 Grouse with regard to the foramen magnum and the occipital condyle ; 

 the former is universally of good size for its owner, subcircular, and 

 without any encroachments upon its margins beyond the condyle. This 

 latter, always sessile, occupies its usual position below the foramen, 

 with its long axis placed horizontally, and so nearly approaching the 

 typical haricot in form that one has no hesitancy in pronouncing it of a 

 renifoj-m outline. In all the Grouse, save Tetrao and Centrocercus, it 

 slightly invades the marginal periphery of the great foramen of the 

 occiput, and in all the excepted genera is more or less shortened trans- 

 versely. 



The second cranial segment constitutes the parietal vertebra, and its 

 elements are shown in the same plate. Fig. 51, where indicators pass 

 through its neural and haemal arches, P. V and P. V^: P. V as the 

 mesencephalic arch, constituted in the comi)lete cranium by the bones 

 P, the parietal or neural spine, when linked with its fellow ; a. .<?., the 

 alisphenoids, the neurapophyses ; m. s., the mastoids, the diapophyses ; 

 and h. s., the basi-sphenoid, the centrnm of the vertebra. The haemal 

 arch we see in the " liyoid," which here shares the same fate of its an- 

 alogue in the occipital vertebra, insomuch as it is ununited to the su- 

 perior arch by either osseous connection or by articulation, for in all 

 living birds the hyoid, the well-known bony supr>ort of the tongue, de- 

 pends entirely upon its muscular and ligamentous connections to retain 

 its relations with the cranium. The manner in which the disjoined 

 neural spine of the parietal vertebra goes to Ibrm the posterior fonta- 

 nelle in the half- grown bird has already been sufficiently dwelt upon. 

 The bone P, as detached in an individual of that age, is quadrilateral 

 in outline, excessively spongy and light, owing to the paucity of com- 

 pact substance over quite a large share of diploic tissue, which is chiefly 

 deposited in a protuberance on its inner table, which protuberance, in 

 union with the fellow of the opposite side of the complete cranial vault, 

 forms two concave surfaces out of the remainder of the sui^erflcies, es- 

 sential portions of the ej)- and ijroseucephalic fossae. 



