182 Scientific Proceedings, Royal Dublin Society. 



borders very irregular ; the so-called sterno-hyoid muscle is in- 

 serted into the whole of the side of the bone. 



The ceratobranchials of the first three arches are hardly dis- 

 tinguishable from one another; that of the fourth arch, however, 

 may be known by a little spur of bone, which projects downwards 

 and inwards from the lower part of the bone. These four bones 

 are slender, rather long and curved, the convexity being down- 

 wards and outwards. Running along the convexity is a deep 

 groove for the branchial vessels and nerves, to the margins of 

 which the gill filaments .are attached. On the inner surface are 

 the gill rakers, which form a double row on the first three arches, 

 and a single otie on the fourth ; the anterior row on the first arch 

 are long and curved, each ending in a single point, which is 

 directed downwards ; in all the other rows they are shorter, not 

 curved, and end in five or six bristly points. The ceratobranchials 

 articulate by a very movable joint with the epibranchials above, 

 and somewhat less movably with the hypobranchials below. 



The ceratobranchials of the fifth arch are shorter than the rest, 

 and constitute the hypopharyngeal bones. They are much broader 

 than the others; the upper expanded surfaces, which. are opposed 

 by the epipharyngeal bones, bear several series of teeth ; they are, 

 connected by ligamentous fibres, in front, to the ceratobranchials 

 of the fourth arch. The under surfaces show a pair of deep 

 grooves for the attachment of the muscular fibres of the oesophagus. 

 The lower ends are the less pointed, and are connected by liga- 

 ment to the urohyal. 



The epibranchials are four small curved bones, which articu- 

 late below with the ceratobranchials, and above with the epipha- 

 ryngeal bones (pharyngobranchials) in a manner to be presently 

 described. 



The anterior is the straightest and the most slender ; the two 

 posterior are greatly curved, and provided with processes, which 

 articulate with each other, being connected below the articulation 

 by a ligament. 



The first and second hypobranchials are small curved bones, 

 grooved on their lower surface, articulating with the copulae and 

 with the first and second ceratobranchials. A. distinct ligament 

 passes from one to the other a little below the middle, and another 

 ligament connects the second to the anterior process of the third. 



