of the Fishery Board for Scotland. 267 



serrated edges, and the upper part of the hiud edge projects beyond the 

 lower half, a little in the saithe and a considerable difference in the cod. 

 In the latter, then, the posterior edge is distinctly beak-shaped. In 

 one or two cases the hind edge of the entopterygoid of the saithe was not 

 serrated, and posteriorly in one case at least the posterior end was of a 

 mitre-shape. In the case of the lythe a small projecting piece has been 

 noticed on the middle of the hind border. 



The pterygoid is bound to the palatine by cartilage. In the cod and 

 saithe the hind end is interlocked with the pterygoid ; in the lythe the 

 union is only by means of connective tissue. 



Preopercidum (pr. Op.). 

 Cod, Fig. 31, PI. X. : Saithe, Fig. 5, PI. X. : Lythe, Fig. 2G, PI. IV. 



The preopercula of the three species are more or less readily dis- 

 tinguished. The flat plate projecting from the anterior edge of the bone 

 articulates with the outer surface of the front process of the hyomandibular. 

 The three species differ in respect to the size of the flat process of the 

 preoperculum. In the cod it is much larger than in the other two ; it is 

 of two parts, an inferior short lobe and a superior long upwards directed 

 triangular lobe. In the lythe the anterior edge of the process is divided 

 by an angle into two nearly equal lobes ; in the saithe this edge is more 

 or less straight and the whole process is of a quadrilateral shape. 



Superiorly the bones difl;er somewhat. In the cod and saithe the 

 anterior superior corner is prolonged into a stout triangular process. In 

 the lythe this projection is not prominent; it forms one side of a notch 

 in the superior border of the preoperculum. 



The anterior edge of the upper half of the bone curves backwards more 

 in the cod and lythe than in tire saithe. 



In the saithe and lythe the preoperculum is much narrower in its lower 

 half than in the superior ; the posterior edge bends anteriorly just at the 

 level of the median articular process. It immediately turns out a little, and 

 then bends anteriorly again, in this way reducing the breadth of the lower 

 half. In the cod the inbend at the level of the median process is hardly 

 noticeable, and the lower half, although it is narrower than the upper, has 

 its posterior edge a regular continuation of the curve followed by the edge 

 of the upper half. 



In the cod the preoperculum is ossified except for a narrow strip of its 

 hind border; in the saithe and lythe a consiierable portion of the 

 posterior region of the bone is transparent, membranous. The ossified 

 anterior half of the bone is thick and its outer surface projects backwards 

 as a broad fold, in this way forming a deep grove which lodges the 

 hyomandibular sensory canal. In the superior part of the bone a posterior 

 wall is furnished to the groove by a short bony ridge. The groove is 

 open in the greater part of its length in all three species, but superiorly it 

 is in the saithe a closed canal, in the cod an open groove, and in the lythe 

 it is either open or partially closed at the lower end of the short ridge. 

 In three lyth(;, viz., female 36 cm., female 47-6 cm., and female 87'8 cm, 

 the groove was open in its entire length, and while in female 82 9 cm., 

 male 86 cm., female 78-4 cm., and female 39'8 cm., there was a little bridge 

 across the groove ; in male 75 cm., female 76'4 cm., and male 71 '4 cm., a 

 considerable part of the upper channel was closed. The projecting fold 

 which forms the groove is in the main portion of the bone supported by 

 three little bony pillars, which may be complete, but usually one or more, 

 or even all, are incomplete in certain preopercula. 



