1926] HaU: Slull of the Bodent Otospermophilus grammurus heecheyi 



385 



Fig. 27. Internal view 

 of cranial cavity of no. 

 29755, (^, equal in age to 

 Q. X 21^. Showing fused 

 post-hypophyseal bars of 

 the petrosals extending 

 transversely across the 

 basisphenoid. 



Explanation: bs., basi- 

 sphenoid; hyps., hypo- 

 physeal fossa ; pet., pet- 

 rosal. 



performed by the posterior clinoid processes of 

 many other mammals. A curved process on 

 each post-hypophyseal bar of the petrosal is 

 raised above the main body of the bar and is 

 directed laterally and posteriorly in the direc- 

 tion of the body of the petrosal. These pro- 

 cesses are usually destroyed in museum skulls 

 that are cleaned. For this reason it is impos- 

 sible to say at just what ag-e these processes 

 appear. The post-hypophyseal bars of the 

 petrosal while not infrequently destroyed more 

 often remain in position. The two bars meet at 

 the median line between O and H. In G the 

 ends lack about 1 mm. of being in contact with 

 each other. At this time each bar is heaviest 

 at the base and tapers to a slender point. 



Auditory ossicles. — The auditory ossicles of Otospermophilus gram- 

 murus, apparently from a fully adult specimen, have been adequately 

 figured by Cockerell, Miller and Printz (1914, figs. 82 to 85). While 

 the Colorado specimen used by them is certainly a different subspecies 

 from the animal found in California and has even been considered as a 

 distinct species (see Grinnell, 1923, p. 321), the auditory ossicles 

 from fully adult specimens from California that I have examined 

 agree with the figures of Cockerell, Miller and Printz (1914, figs. 

 82 to 85), This agreement, however, would not appear to be signi- 

 ficant with respect to indicating that the forms from the two regions 

 are conspecific, for, from the observations of the last-named authors 

 (1914, p, 369), it appears that the auditory ossicles of the Sciuridae 

 do not afford characters of systematic value in closely related species. 



The form and relation of these bones in age B are shown in figure 

 26. At this time all are cartilaginous. The incus has assumed more 

 nearly the adult condition than has either the malleus or stapes. Both 

 the malleus and incus are in contact with Meckel's cartilage. That 

 portion of the malleus which will form the processus cephalicus and 

 the head, lies between the tympanic and Meckel's cartilage. The manu- 

 brial portion lies against the membrane in which the tympanic ossifies 

 but is not attached to the tympanic membrane if the same be present. 

 The stapes at this time seems to be no more columniform than in the 

 adult. The convexity of the footplate is not apparent. The stapedial 

 process is not present. The crura are well developed near the head, 



