of the Fishery Board for Scotland. 



63 



posing the vertebral column for this purpose, in a large number of fish, 

 made it prohibitory. A modification was therefore adopted, and a point 

 chosen, which was almost, if not altogether, of equal fixity with the end 

 of the vertebral column. 



Though the leugtM_of the bony support (hypural bones) of the caudal fin 

 rays is not invariably of the same size in equal-sized fish, it is subject to 

 fewer variations, and these variations, from the shortness of the total (about 

 10 mm.), are of very small amount. Now, as it is close to this that the 

 silvery sub-epidermic layer covering the body terminates, thispoint — 

 the few scales, if present, being scraped ofi" — forms a well-marked one for the 

 position usually called the base of the caudal fin, though it does not actually 

 correspond to that position, and the whole length from the tip of the closed 

 lower jaw to this point was therefore generally used as the standard by 

 which the other measurements were to be compared. This is one of those 

 minor details in which I have difi'ered from Heincke, who employs the 

 total length including caudal fin, but which has not a serious effect on the 

 result, except in so far as it affects comparison of details. Then the 

 exact fixture of the point which I have described does away with Heincke's 

 objection to the indefiniteness of the terms ' without caudal fin' or 'base 

 * of caudal fin.' The fish on arrival were placed on a sheet of paper in 

 such a position that a straight line would run through the tip of the 

 closed lower jaw and the fork of the tail. A pencil was carried round the 

 fish, so that a rough but fairly accurate life-size sketch was formed of it, 

 useful in the event of its being afterwards necessary to have a general 

 idea of its size and shape. The following points were then accurately 

 marked off, and the measurements of length of head, positions of fins, &c., 

 made along the longitudinal central line to perpendiculars drawn to it 

 from the various points, except in the case of length of base of fin, the 

 measurements" of which were made along the actual fin : — 



1. Anterior end of mandible. 



2. Anterior end of premaxilla. 



3. Centre of eye. 



4. Back of head (of supraoccipital bone, which has an almost straight 

 posterior border) ; this forms a much better standard of length of head 

 than the length of the side of the head, which includes the opercular 

 bone — liable to considerable alteration in position from various causes. 



5. Base of anterior ray of dorsal fin. 



6. Base of posterior ray of dorsal fin. 



7. Termination of tail (as described above). 



8. Tip of caudal fin. 



9. Base of posterior ray of anal fin. 



10. Base of anterior ray of anal fin, (This point has nearly always the 

 same relative position with regard to the vent, any difference being more 

 apparent than real, occurring especially during the spawning season from 

 the enlarged and tumid condition of the latter, and from its contracted 

 state in the spent condition.) 



11. Base of first ray of pelvic fin. 



12. Base of first ray of pectoral fin. 



13. End of branchiostegal membrane. 



14. Articulation of lower jaw. 



Notes were also kept of the number of rays in the five fins, of the con- 

 dition of the reproductive organs, of the date and place of capture, and, 

 in several instances, of the number of keeled scales. The whole of the 

 measurements were taken in millimetres. 



For the purpose of comparison it was necessary that the whole of the 

 measurements thus procured should be reduced to a ratio of some common 



