THE THREE^SPINED STICKLEBACK 243 



However, intermediate forms may occur, as for 

 example specimens exhibiting the semiarDiatiis type 

 on one side of the body and the trachuriis type 

 on the other, and the study of a large series of 

 specimens shows that there is a continuous grada- 

 tion from the gymnurus to the tracJiurus armature. 



In the British Isles it may be generally stated 

 that in inland localities all the Sticklebacks are of 

 the gymnurus type ; this applies to a number of 

 examples from various parts of Ireland sent to me 

 by Mr. Robert Patterson, to a series of specimens 

 from Newark for which I am indebted to Mr. R. 

 Littler, and to some from Shrewsbury, given to me 

 by Mr. H. E. Forrest, as well as to many others 

 from various parts of Britain. Sticklebacks from 

 estuaries, or places not far from the sea, exhibit a 

 remarkable inconstancy in the development of the 

 plates, all gradations from the gyimiurus to the 

 tracJiurus type being often found in the same shoal. 

 In the sea, at any rate on the northern coasts of 

 our islands, the trachurits type prevails. 



In the northern part of its range the Three- 

 spined Stickleback is typically a marine species with 

 strong dermal ossification, the series of bony plates 

 complete and the individual plates deep, the caudal 

 keel strong, the ectocoracoids long and the naked 

 areas above them and in front of the pectoral fins 

 consequently large, the pelvis long and lanceolate, 

 without an anterior notch, and the fin-spines either 

 long or strong. I have compared examples of this 

 type from the Shetlands with others from Alaska 

 and Puget Sound, and I am unable to detect any 

 difference. In these high latitudes a reduction of 

 the dermal ossification is found in freshwater 



