On Variations in (he Pectoral Fin. 319 



lower half of venation ocliraceous^ the upper half black. 

 Head (including eyes) considerably broader than base of 

 mesonotum ; eyes protuberant, moderately stylate ; length 

 of head about half its width between eyes and almost as 

 long as pronotum^ the lateral angles of which are rounded ; 

 basal areas of cruciform elevation strongly obliquely striate ; 

 opercula transverse, not passing base of abdomen ; anterior 

 femora strongly spined beneath. 



Long., excl. tegni., (^ 43 mm. ; exp. tegm. 127 mm. 



Ilab. Central Brazil: Chapada, 2600 feet {A. Robert, 

 Brit. Mus.). 



XXXVII. — The Changes and Variations in the Position of the 

 Pectoral Fin during Development. By H. H. SwiNNERTON, 

 D.Sc, University College, Nottingham. 



For some time I have been investigating the development 

 of the Teleostean pectoral fin skeleton *. This has brought 

 into my hands an extensive series of developmental stages of 

 the three-spined stickleback {G aster osteus aculeatus). Being 

 struck by the difference in the relative positions of the fin 

 in the youngest and oldest stages, I measured numerous 

 specimens of all ages, with a view to ascertaining definitely 

 if this indicated a real shifting during development. 



The investigation above referred to brings out the fact that 

 the glenoid border tends to rotate from a horizontal to a 

 vertical position during development. This rotation is 

 around the ventral end of the border. Consequently this 

 point seemed the most suitable one from which to take 

 measurements. Two were taken in each case, viz. from this 

 point to the tip of the snout and to the hinder extremity of 

 the notochord. 



In fig. 1 (p. 820) the vertical line represents the distance of 

 the pectoral fin from the tip of the snout, the horizontal that 

 from the extremity of the notochord. The oblique lines give 

 the average position of the fin at various stages. To obtain 

 these the specimens were put together into groups containing 

 individuals differing in length only 1 mm., and the average 

 was found for each group. Gaps are left because the material 

 proved insufficient to give trustworthy results at these points. 



The shortest line represents the average position of the fin 



* My thanks are due to the Government Grant Committee for aid in 

 carryiiif,' on this investigation. 



