UNGULATA. 



16: 



The periotic and tympanic are ankylosed together, but not 

 with the squamosal. The former has a wide but shallow 

 floccular fossa on its inner side, and sends backwards a 

 considerable " pars mastoidea " which appears on the outer 

 surface of the skull {Per) between the post-tympanic process 

 of the squamosal and the exoccipital. The tympanic (Ty) 

 forms a tubular meatus, directed outwards and slightly 

 backwards. It is not dilated into a distinct bulla, but ends 

 in front in a pointed styliform process. It completely 

 embraces the truncated cylindrical tympanohyal (///), which 

 is of great size, corresponding to the large development of 

 the whole anterior arch of the hyoid. 



The stylohyal (Fig. 57, sh) is of great 

 size, compressed, and expanded at the 

 upper end, where it sends off a triangular 

 posterior process. Below the stylohyal, 

 and usually becoming ankylosed with it, 

 is a small nodular bone (epihyal), and 

 then the arch is completed by a short 

 cylindriform ceratohyal {ch). The basi- 

 hyal (bh) is rather flattened from above 

 downwards, arched with the concavity be- 

 hind, and sends forwards a long, median, 

 pointed, compressed " glossohyal " pro- 

 cess. The thyrohyals (th) are compressed 

 bars projecting backwards from, and in 

 adult animals completely ankylosed to, 

 the lateral extremities of the basihyal. 



Each ramus of the mandible has a 

 long, straight, compressed, horizontal 

 portion, graduallv narrowing towards Fig. 57.— Superior surface 



. . of hvoid bones of horse, 



the symphysis, where it expands laterally J. sk stylohyal ; ch cera- 



/ r J . ' , , , • tohyal; bh basihyal; th 



to form with the ankylosed opposite thyrohyai. 



