296 Part III — 'Eighteenth Annual Report 



IV. Length of Snout ; from zero to the point where the perpendicular from 



centre of the eye cuts the axis, I Sn. 



V. Lateral Length of Head ; from zero to the point where the perpendicular 



from the hind edge of the operculum cuts the axis, 11 H. 



VI. Distance of Pectoral Fin ; from zero to the point where the perpendicular 



through the proximal end of the first ray of the fin cuts the axis, d Pf. 



VII. Distance of Ventral Fin ; from zero to the point where the perpendicular 



through the proximal end of the first or outer ray of the ventral fin cuts the 

 axis, (I Vf. 



VIII. Distance of First Dorsal Fin ; from zero to the point where the perpendicular 

 from the anterior edge of the base of the first dorsal cuts the axis, d 1 D. 



IX. Distance of Second Dorsal Fin ; from zero to perpendicular from the 



anterior edge of the base of the second dorsal, d 2 D. 



X. Distance of the Anus ; from zero to perpendicular from the hind edge of the 



anus, d A. 



XL Distance of Anal Spine ; from zero to the perpendicular from the anal spine, 

 d As. 



XII. Length of Skull; from the edge of the occipital foramen to the tip of the 



ethmo-palatine boss, / Sk. 



XIII. Length of Mandible ; from the extremity of the articular to the tip of 

 the mandible, I M. 



XIV. Length of Dentary ; the length of the ossified portion of the dentary, I De. 



XV. Length of the Tail ; measured from the anterior edge of the 31st vertebra 



to the tips of the shortest rays in the fork of the tail, I T. 



XVI. Length of Superior Lobe of the Caudal Fin ; measured from the superior 

 hind edge of the 29th vertebra to the extreme tip of the lobe of the fin, I Cr. 



In addition to the preceding, the following characters which admit of 

 enumeration were chosen : — 



XIX. The Number of Vertebra. 



XX. The Number of Rays in the First Dorsal Fin. 



XXI. ,, ,, ,, Second 



XXII. ,, ,, ,, Anal Fin. 



XXIII. ,, Dorsal Finlets. 



XXIV. „ Anal 



XXV. Weight. 



The Length of the fish was taken as the distance from the tip of the 

 snout to the middle of the fork of the tail. This standard was adopted 

 by Gars tang.* 



Mode oP Measurement. 



Since there are not on the mackerel external marks by which the origin 

 of any of the fins may be sharply defined, it was necessary to select certain 

 artificial points which would enable the measurements to be made all 

 through with some degree of uniformity. The distances on the body were 

 measured from the one origin — viz., the tip of the snout. To mark the 

 beginning of the pectoral fin a pin was inserted at the angle 

 between the base of the first ray and the scapula (3, fig. 3) ; the 

 same point was used in the measurement of the length of the pectoral fin, 

 the other limit being the extreme tip of the same ; for the beginning of the 

 ventral fin a pin was inserted at the angle between the first or outer ray of 

 the ventral fin and the ventral surface of the body (4, ib.) ; the beginning 



* Garstang, "On the Variation, Races, and Migrations of the Mackerel (Scomber 

 scombrus)," Journal Marine Biological Association, N.S., Vol. V., No. 3. 



