GASTEROSTEUS ACULEATUS. 79 



constant, the conclusions of Cuvier would still be maintained, and numerous 

 species of Sticklebacks of the three-spined type be recognised, which would 

 show remarkable modifications with geographical distribution. But since 

 the characters are very variable there has been a tendency to push the doctrine 

 of specific variation as far as possible, and group under the name Gasterosteus 

 aculeatns all the three-spined Sticklebacks of Europe and America. If we 

 contrast the extreme terms of this series as represented by the Gasterosteus 

 aculeatus with lateral armour extending from the head to the tail, with a 

 form like Gasferosteus argyropomus, in which the sides of the body are 

 entirely free from plates, then no one could hesitate;, especially keeping in 

 view the short first dorsal spine of the latter form, to regard it as a separate 

 species, which, being limited to Italy, might be looked upon as a geographical 

 representative. But when we find in the fresh waters of France every in- 

 termediate variation in lateral armature, the idea of separating the Italian 

 type as a species fails. There are only two varieties known in Italy, that 

 to which we have just referred, and the partly-armoured Gasterosteus brachy- 

 centrus ; and both these are united by Professor Canestrini as furnishing 

 the Italian type of Gasterosteus aculeatus. And Dr. Steindachner, after study 



Fig. 29. GASTEROSTEra ACULEATUS, VARIETY BKACHYCENTRUS. 



of the Spanish types, adopted the conclusion that the Gasterosteus brachy- 

 centrus must also be merged in the G. aculeatus, 



As a type, however, which has been generally regarded as a species 

 by the writers of Great Britain and the Continent, we have thought it 

 desirable to point out its more distinctive variations and characters (Fig. 29). 

 All the fin-spines are shorter, and the first dorsal spine begins over or 

 somewhat behind the base of the pectoral fin, instead of in front of it. The 

 head is longer than in the Common Stickleback, and longer in the male 

 than the female. The greatest height is never equal to the length of the 

 head, even in the female at the time of spawning. The diameter of the 

 eye is a quarter of the length of the head; it is separated from the other 

 eye by its own diameter. The mouth is more oblique, and the mandible 

 projects over the pre-maxillary, especially in the males. The dentition 



