180 Scientific Proceedings, Royal Dublin Society. 



layers are attached to the ridges on the under surface of the frontal 

 bone, and are continued to the prefrontal, and, fusing together 

 below, thus bound a prismatic space between them and the bone, in 

 which the olfactory tracts lie. At its attachment to the para- 

 sphenoid and prootic the interorbital septum is continuous with 

 the infraorbital membrane (above described). 



The Hyoid and Branchial Arches. — A typical branchial arch 

 consists of four pieces, nanied, from above downwards, pharyngo- 

 branchial, epibranchial, ceratobranchial, and hypobranchial. In 

 some fishes [e. g. salmon) the three foremost arches are found 

 nearly in this condition, but in many others the pharyngobran- 

 chials, which usually expand and bear teeth, show a great ten- 

 dency to become distorted and fused together, and to have their 

 articulations displaced : moreover, some of the hypobranchials 

 may be wanting, the most constant of these four bones being 

 the ceratobranchial, which is the only representative of the fifth 

 arch. The hypobranchials are connected in the middle line by a 

 series of small bones, or cartilages, the copulse, the anterior of 

 which is called the entoglossal, and the posterior the urohyal ; the 

 hyoid arch is built on the plan of a branchial arch, but the parts 

 are more numerous, and peculiarly modified to perform different 

 functions. The uppermost part of the arch, the hyomandibular, 

 has been already described ; also the symplectic (which forms a 

 sort of link between the hyoid and mandibular arches). The re- 

 maining parts are the stylohyal, epihjal, ceratohyal, and two 

 hypohyals. Below the hypohyals is a bone, often passed over 

 unnoticed in text-books, for which the name of basibranchiostegal, 

 applied to it by Parker, is perhaps the most .convenient. 



The stylohyal (PL VII., fig. 15) is a small rod-like bone, which, 

 articulating with the triangular intercalary cartilage above, passes 

 downwards internal to the preoperculum, and articulates with the 

 epihyal; the joint so formed is supported behind by an articular' 

 cavity on the inner side of the interoperculum. 



The epihyal is a triangular piece of bone, the apex articulating 

 with the stylohyal in a movable joint, and the base immovably 

 with the ceratohyal ; it is separated from the latter bone below 

 and externally by a strip of cartilage, but joins it internally by a 

 spKntery serrated suture. 



The ceratohyal, nearly three times as long, as the preceding 



