310 BULLETIN UNITED STATES GEOLOGICAL SURVEY. [Vol. VI. 



in tliis monograpli we will omit, as we have in former ones now published, 

 any detailed description of the osseous elements of the ear, or the respir- 

 atory tube, small sesamoids, or such tendons as may ossify in the extrem- 

 ities. The hyoid as an arch of one of the cranial vertebrae evidently is 

 not included in this category, and will in consequence receive the 

 attention it undoubtedly deserves in its proper place. 



The study of the bones entering into the cranial vertebrae has been 

 initiated at a stage in the chick's life a few days after birth, and their 

 relation to each other and their development carried up to the adult 

 bird. It will be observed after a glance at the writer's drawings in 

 Plate V that he has chosen the young of that grand old prairie-loving 

 Grouse, Centrocercus iirophasianus, as an example of the growth of the 

 skull from the time above referred to in the Tetrceonince. 



In this plate the first three figures show respectively the skull of the 

 young of the Sage Cock a few days after the parent has led it from the 

 nest : 47 fi"om above, 48 lateral \ieW5 with mandible, and 49 from below, 

 the mandible removed. 



Fig. 50 shows the bird in August of the same year, and Fig. 51 the 

 disarticulated skull of the same, whereas in the next plate we observe 

 the skull of an old cock of the same species that has, no doubt, trod 

 the prairie for many a season. (Fig. 52). 



In these birds the greatest amount of difference exists in point of size 

 among the sexes and in individuals of various ages of the same sex ; so 

 we naturally find a corresponding amount of ditference in the sizes of 

 their crania. 



Fig. 52 is the skull of an exceptionally large adult, ^ , chosen from a 

 bevy of several hundred others, with a view of affording the reader the 

 opportunity of seeing the i^roportions this Grouse may attain, as far as 

 this part of its skeleton is concerned. This peculiarity seems to be con- 

 fined to Centrocercus^ and does not obtain with the other varieties, they 

 seemingly arriving at maturity of growth at a much earlier period of 

 their existence. Tetrao obscurus may form an exception to these re- 

 marks, but it is certain that it is not by any means so striking a char- 

 acteristic in this bird. Another interesting point to be observed here, 

 that no doubt has forced itself upon the reader since his inspection of 

 the plates already introduced, is the unusual length of time that the 

 original bony segments of this Grouse's head retain their individuality, 

 over others of the class. This is indeed so, and in birds of one or two 

 years of age, if we exclude the epencephalic arch of the occipital ver- 

 tebra, it is not an unusual occurrence to be able to distinguish all the 

 sutural boundaries among the remaining elements, and these appear to 

 be persistent when applied to the nasals and the premaxillary bone of 

 very old birds. We are all well aware that this rule holds good in the 

 common barn-yard fowl. 



Students of the works of that eminent anatomist and observer, Owen, 

 will remember that in his Comparative Anatomy and Physiology of 



