314 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Tol.Yl. 



Superficially, these elements are smooth and convex, and in the adult, 

 after consolidation, exhibit some faint evidence of a parietal eminence 

 on either side, — more marked elevations, however, occurring in the 

 spine of the vertebra bej^ond, immediately anterior to the suture termed 

 in Anthropotomy the " coronal." With the exceptions of the tympano- 

 mastoidal articulation and the connections between the mastoids and 

 petrosals, the majority of the articulations of this vertebra in the mid- 

 aged bird may be classed among the variety known and described in 

 works upon human anatomy as the " squamosal," — the parietals being 

 bevelled above to accommodate themselves to the frontals. 



The alisphenoids are separated from each other mesially by nearly 

 half a centimeter; above they meet the frontals, below the basi-sphe- 

 noid, and laterally the mastoids, — the lower and outer angles almost 

 reaching the cup-shaped articulation for the tympanies. This segment 

 seems to ossify from its borders towards the centre, leaving a foramen 

 that is eventually closed in. On its mid and lower border it presents 

 for examination the half of the "foramen ovale," which is completed by 

 meeting the centrum of the vertebra. It is for the transmission of the 

 trigeminal nerve into the orbital cavity. Laterally there is developed a 

 quadrate apophysis (the parapophysis of the vertebra ?), which joins 

 with a similar, subsequently scale-like process coming from the mas- 

 toid, resulting in a foramen, cordate in outline above, elliptical below, 

 between them, giving passage to the fibres of the temporal muscle, that 

 is markedly characteristic of the Tetraonidcc. 



Below the point of union this apophysis is triangular, with its apex 

 pointing forwards and downwards, flat, with its inner surface looking 

 forwards, upwards, and inwards. Internally, the alisphenoid is deeply 

 concave. (Plate VI, Fig. 52, and other skulls illustrating this paper). 



The external appearance of the mastoid is well shown in Plate V, Fig. 

 50, and as m. s.. Fig. 51. Internally, the half-cells observed close in by 

 the aid of similar excavations in the segments of the occipital vertebra, 

 the acoustic capsule ; and a double-concave surface, assists in forming- 

 cranial fossse. 



We now come to examine the ornithic characters of one of the most 

 interesting segments of the bird-skull, the centrum of the parietal 

 vertebra, well termed by most ornithotomists and general anatomists as 

 the basi-sphenoid. At an early date in the life of the chick {Centrocer- 

 cus and others) this bone becomes confluent with the centrum of the 

 frontal vertebra beyond ; this coufluence takes place, if we may be 

 allowed to differ with such high authority as Owen, who makes the 

 rather sweeping assertion " that the pit for the pituitary body marks 

 the boundary " (Comp. Anat. and Phys. of the Vert., vol. ii, p. 45) in the 

 following manner, and the sutural trace is yet discernible in young birds 

 of the Family under discussion (Plate V, Fig. 51). The pre-sphenoid 

 lies beneath a tuberous process projecting anteriorly from the latter 

 bone, reaching nearly as far back as the carotid foramina. The com- 



