170 Scientific Proceedings, Royal Dublin Society. 



occipital, forms a bony hoop, whicli supports the large otolith, being 

 applied against the posterior end of its inner convex surface.^ 



The opisthotic is a thin bone, which is bent in such a manner 

 as to present a notch posteriorly, which is occupied in the recent 

 state by the anterior end of the kidney and the branches of the 

 vagus nerve. From the upper margin a process projects inter- 

 nally, which articulates with the epiotic. The pterotic articulates 

 with the whole upper margin of the bone, and with the front of 

 the above-mentioned process. In front it articulates with the 

 prootic, and below with the basioccipital, and slightly with the 

 parasphenoid. The posterior margin bends slightly inwards, and 

 articulates with the esoccipital (which also joins the lower edge of 

 the epiotic process). The upper limb of the notch supports on its 

 inner surface the external limb of the post-temporal. Near the 

 junction of the upper and anterior borders the bone is perforated 

 by a foramen for the glosso-pharyngeal nerve. The cerebral sur- 

 face is applied against the labyrinth, especially the external semi- 

 circular canal. 



The pterotic forms the greater part of the lateral ridge or 

 parotic process of the skull. Its outer surface is excavated, 

 affording origin to a group of muscles, which raises the gills. In 

 front it articulates with the sphenotic and prootic bones, which 

 complete, with it, the articular cavity for the hyomandibular, in 

 the formation of which the pterotic takes the largest share. Below 

 it articulates with the opisthotic. It overlaps the sphenotic on the 

 upper surface of the skull, but is overlapped by the latter bone on 

 the lateral surface. It also articulates on the roof of the skull 

 with the epiotic and parietal, and presents a thin ridge," which 

 forms a backward continuation of the outer ridge of the frontal. 

 Its cerebral surface forms a bony ring around the external semi- 

 circular canal. In front of this a process, going downwards and 

 forwards, meets an ascending process of the prootic. This forms 

 another ring around the anterior vertical semicircular canal and 

 front of the vestibule, and the space in which they lie is closed 

 above by the sphenotic. 



^ The otoliths are not described here, as they beloDg more properly to the anatomy 

 of the ear than to general osteology. 



2 Spoken of above as the " inner ridge of the pterotic." 



