228 Part III.—Twentidli Animal Rqwrt 



III.— A COMPAEISON BETWEEN THE COD {Gad^is callarias, 

 Linn.), THE SAITHE ((^ar/ws virens, Linn.), AND THE LYTHE 

 {Gadus pollacMus, Linn.), IN RESPECT TO CERTAIN EX- 

 TERNAL AND OSTEOLOGICAL CHARACTERS. By 

 H. Chas. Williamson, M.A., D.Sc. 



(Plates IV. XL) 



INTRODUCTION. 



This research has been undertaken as part of a review of the species of 

 tlie Cod family. Tliat a revision of the Gadidfe is required is patent to 

 anyone who has had to deal with some of the rarer forms. The 

 re-examination of the different species was not in the first place under- 

 taken with a present expectation that a reconstruction of the classification 

 may l)e required, but rather with the view of finding a certain number, 

 as few as possible, of external characters which would sufhce for ready 

 discrimination. 



While for the mere diagnosis of species the gauging of the external 

 characters would suflrce, the study of the osteology, of which these 

 characters are the external expression, follows as a natural sequence. 

 The osteological part of the paper has been limited to the bones of the 

 head. 



No general deductions are made in this paper regarding the classi- 

 fication, since these would only result when similar data regarding the 

 other Gadid species are available. 



The fishes examined were obtained from the following sources. Most 

 of the cod and saithe were procured from the Aberdeen Fish Market ; one 

 or two specimens of each were captured in the salmon-nets in the Bay of 

 Nigg. Some of the lythe, which is a comparatively rare fish, were 

 obtained from the two sources of supply mentioned above. Several were 

 got by Dr. Fulton, during his trawling experiments off the coast of 

 Aberdeenshire, and in the Moray Firth ; two from St. Andrews were 

 furnished by Professor M'Intosh, while a considerable number were sent 

 from Girvan (Firth of Clyde) by Mr. R. Duthie, Fishery Officer. 



External Characters. 



In the selection of the external characters by which the species are to 

 be compared, the observer has an absolute freedom. Certain characters 

 demancl immediate attention by their prominence as distinguishing marks 

 between species. But, at a rule, any and all measurements are of value 

 as tests either of agreement or of difference. 



A proportion of the fishes upon which the measurements of the 

 external characters were made were immature specimens. 



Owing to the small number of fishes included in this investigation the 

 measurements have not been treated by the Biometric method. The 

 three forms have been compared by means of the averages of the 

 characters. 



The fiiUowing external characters have been adopted : — 



I. Girth, at three points, viz. — 



(1) PectoraJ, just behind bases of ventral fins. 



(2) At anus. 



(3) At root of tail. 



