264 Part III. — Tirentieth Annual Report 



elevated border is small and distinctly separated from the articular 

 surface ; in the saithe and lythe it is massive. The superior corner of 

 the articular is broader in the lythe than in the saithe. The edges of the 

 groove in the articular which holds part of the hyomandibular sensory 

 canal are in the saithe and lythe low ; in the cod they are prominent. 



In front the articular fits into the hollow dentary; behind it articulates 

 with the quadrate and is bound to that bone and the preoperculuni by 

 ligaments. The angular is connected by means of a strong ligament to the 

 interoperculum. 



Hyomandibular Arch. 



Hyomandibular (Hy. M.). 



Cod, Fig. .36, PI. X. : Saithe, Fig. 12, PI. X. : Lythe, Fig. 11, PI. IV. 



The lujomandihular has three large processes ; the hind one, which has 

 a hollow extremity, articulates with the operculum by means of a liga- 

 ment ; the median, a boot-shaped expansion, is firmly attached to the upper 

 part of the anterior border of the preoperculum, while the anterior process 

 is fixed behind to the median flat plate-like process on the front edge of 

 the preoperculum, and anteriorly to the metapterygoid. The syra^ilectic 

 is united by ligament to the union between the anterior process of the 

 hyomandibular and the preoperculum. This cartilage is lodged in the 

 hollowed extremities of the processes of the hyomandibular and sympiectic. 



The hyomandibulars of the three species differ to a considerable extent 

 from one another. In the saithe and lythe the median boot-shaped 

 process is quite free from the hind process except at its base ; in the cod 

 the heel and back of the boot are united to it by bone, and the uniting 

 bone-plate forms a ridge running out to the extremity of the hind process. 

 Moreover, the toe of the boot is pointed almost vertically downwards in 

 the cod, while in the lythe and saithe it is directed downwards and 

 forwards. 



The anterior process is of two parts : the hind portion has a hollow 

 extremity and lodges a ligament which binds it to the sympiectic ; the 

 fore part is irregular and serrated for suture with a complementary surface 

 on the metapterygoid. In the cod this fore irregular portion is nearly 

 as broad as the hind articular part. From its anterior border there rises 

 a bony wall which forms with a broad expansion from the main body of 

 the bone an arch which overhangs the proximal extremity or origin of the 

 front process. This arch joins the median boot-shaped process. The hollow 

 beneath the arch lodges certaiii of the muscles pertaining to the mandible. 

 In the saithe and lythe homologous structures are present, but they do 

 not form a distinct arch as in the cod. 



Superiorly the bone is rounded into a large knob which fits into the 

 socket on the squamosal. On the hind edge of the posterior process 

 there is a thin lamina of bone uniting it to the main articular knob. It 

 runs along the former as a sharp high ridge, which narrows rapidly to 

 vanish at the extremity. In the lythe it extends to the end of the 

 process ; in the saithe it usually stops a little short of the same ; in the 

 cod it is small and does not extend along much more than half the length 

 of the process. 



If the bones are looked at from in front (as they lie in the fish), it is seen 

 that the prominent ridge arising from the anterior border of the front 

 process gives a great breadth of triangular shape to the fore edge of the 

 bone in the cod. In the saithe and lythe the anterior edge is narrow. 



