270 Part III. — Tu^entieth Annual Report 



of the lythe occupies an intermediate condition between those of the cod 

 and saithe. In the region where the first three branchiostegal rays are 

 attached to the internal surface of the ceratohyal, the ventral outline of 

 the bone bends upwards, then runs forwards, to bend slightly downwards 

 before reaching the extremity of the bone. la the cod there is here then 

 a fairly deep concavity in the ventral outline, in the saithe a slight 

 inbending, and in the lythe an intermediate condition. 



In the saithe there is in this region a strong ridge coming down to the 

 ventral edge. This gives to the bone in front of it a distinct depression 

 which is not prominent in the lythe and cod. 



In the case of the epihyals no readily distinguished points of difference 

 were made out. 



The stylohyal of the lythe is longer and broader at its base than that 

 of the saithe in fishes where the ceratohyal and epihyal, along with the 

 two basihyals, form structures of equal length. Tinder similar circum- 

 stances the stylohyal of the cod is of about the same length as that of the 

 lythe, and greater than that of the saithe. 



Urohyal (u. Hy.). 

 Cod, Fig. 50, PI. X. : Saithe, Fig. 25, PI. X. : Lythe, Fig. 16, PI. IV. 



The differences between the urohyals of the three species are apparent 

 at a glance. The head of the urohyal is deeply cleft, and in the cavity 

 is attached the ligament binding it to the upper basihyals and to the first 

 basibranchial. On either side of the anterior edge there is attached a 

 strong broad ligament binding it to the basihyals on either side. 



In general shape the urohyal of the saithe is roughly rectangular, with 

 the cleft head projecting from the upper proximal angle. In the cod the 

 bone is roughly triangular, while in the lythe the urohyal differs from 

 that of the saithe by the greater slope of the anterior edge, and by the fact 

 that while the blade of the bone remains in the saithe of about the same 

 breadth in almost its whole length, in the lythe it tapers rapidly and ends 

 in a pointed extremity. In the saithe the posterior extremity is rounded in 

 outline. The end of the urohyal is serrated in the saithe and lythe : in 

 the lythe the serrations are fewer and deeper. In the cod the posterior 

 border of this bone is smoothly rounded, with superiorly some ridges and 

 blunt processes to which ligaments are attached. The urohyal is 

 traversed longitudinally by three ridges which radiate from a point 

 situated a little below the middle of the front border. The radiating 

 ridges are more prominent in the lythe than in the cod and saithe, The 

 urohyals are cleft longitudinally at the inferior proximal corner. The 

 anterior end of the urohyal of the saithe is rounded, and is on the whole 

 vertical to the ventral edge of the bone ; in the cod this edge slopes back 

 ward slightly, and in the lythe very much. 



Branchiostegals Brst.). 



Cod, Fig. 49, PI. X. : Saithe, Fig. 24, PI. X. : Lythe, Fig. 15, PI. lY. 



The seven branchiostegals are divided into two groups. The first 

 three rays articulate in depressions on the inner side of the ceratohyal, 

 while the 4th to 7th inclusive articulate in three broad depressions on the 

 outer surface of the bone. The 6th and 7th articulate with one and the 

 same depressed area. The first ray is the smallest, and the rays follow 

 one another in regularly ascending series. It is thus easy in the event of 

 a complete set of branchiostegal rays of any one fish to arrange them in 



