274 Part III. — Twentieth Annual Report 



short broad shallow groove on the superior half only of the posterior 

 surface. On the fourth the groove occupies the upper two-thirds. The 

 fourth ceratobranchial is readily distinguished from the first (and also 

 from the second and third) by the presence of a large irregular projection 

 on its lower half. It rises from the posterior surface, and is directed 

 downwards. The groove in the hind surface of this bone turns inward 

 round the base of this expansion. The first ceratobranchial has a 

 longitudinal depression or groove on its anterior edge. 



The second and third, which are readily distinguished from the other 

 two, are not very easily separated from one another. If, however, the 

 bones are held with the posterior grooved surfaces facing the observer, it 

 is seen that while in the third ceratobranchial the groove remains, in the 

 loirer third of the bone, of one width, in the second, in that part of the 

 bone the groove gradually widens, attaining its greatest width at the 

 inferior extremity. Moreover, the groove on the third is seen to bend 

 forwards and outwards near the lower extremity of the bone, exposing to 

 view a considerable portion of the quill-like side of the articular cavity. 

 The lower end of the tliird is of greater diameter than that of the 

 second. 



Epihranchials (ep. Br.). 



Lythe, Fig. 21, PI. IV. 



Of the epihranchials, of which there are four, the third and fourthare 

 tri-radiate ; the second suggests in its form that shape. The first is a 

 lonf^ narrow bone having a thick inferior end and a narrow upper 

 extremity ; it has about the middle of its length a short triangular down- 

 ward-directed process, which may be regarded as a rudiment of the third 

 arm noticed in the third and fourth bones. The epihranchials articulate 

 inferiorly with the four ceratobranchials, and above with the upper 



pharyngeals. u 4.1, ^ * 



The second epibranchial is much nearer a tri-radiate form than the hrst ; 

 on its hind border there is a large triangular expansion. This projection 

 is situated on the lower half of the bone. 



The third epibranchial is distinctly tri-radiate ; the three arms are of 

 about equal length. It is, moreover, distinguished from the other 

 epihranchials by a toothed area which it alone has on its anterior edge. 

 The end of each ray serves to lodge the end of a ligament. The thickest 

 extremity is superior, and the end directly opposite is that which articulates 

 with the third ceratobranchial. The third or hind ray, which is directed 

 downwards and which is united to the lower arm by a thin plate of bone, 

 articulates with a corresponding ray of the fourth epibranchial. The 

 fourth epibranchial, also tri-radiate, has, however, a short hmd arm, which 

 is directed downwards. 



Upper Pharyngeals (up, Phr.). 



Lythe, Fig. 21, PL lY 



The . ,.per pharyngeals are three in number ; they fit together to form 

 a rounc' toothed area situated on each side above the fauces. 



The first or most anterior has a somewhat sickle-shaped toothed area ; 

 it hps at its anterior corner a short process which articulates with the first 

 epibranchial. On the under surface of the opposite corner there is a 

 hollow which serves for articulation with the superior extremity of the 



