54 Messrs. T. and A. Scott on some 



Posterior antennae three-jointed, second and third joints elon- 

 gate and subequal, the first short, about half as long as the 

 second; secondary branch very small, one-ininted, and arti- 

 culated to the lower distal end of the first joint of the primary 

 branch. Mandibles narrow, cylindrical, armed witli a few 

 moderstely long teeth; basal portion of mandible-palp stout, 

 somewhat dilated, and furnished with a small one-jointed 

 branch at the apex (fig. 4, PI. VI.). Posterior foot-jaws 

 small, the first two joints moderately stout, the last very 

 small and forming the base of a slender and comparatively 

 short claAV (tig. 26, PI. V.). The first pair of swimming-feet 

 reseml)le those of Mesochra Lilljehorgiij Boeck, except that 

 the end joints of tlie inner branches are proportionally longer, 

 being equal to about half the length of the first joint (fig. O, 

 PI. VI.). Outer branches of the second, third, and fourth 

 pairs elongate ; inner branches short, two-jointed (fig. 6, 

 PI. VT.). Fifth pair foliaceous, small; the basal joints are 

 subquadrangular, and their width equal to nearly twice the 

 length ; but the slightly produced inner portion is triangular 

 and furnished with two setai on the inner margin and one at 

 the apex ; secondary joints subquadrate, being nearly as 

 broad as long, and bearing six setse, arranged at slightly 

 irregular intervals round the outer margin and end ; the 

 second seta, counting from the inside, is much longer than 

 any of the others (fig. 7, PL VI.). Caudal stylets short, 

 length rather greater than the width, each provided with 

 several seta?, the principal seta being equal to nearly three 

 fourths of the length of the animal. 



No males of this species were observed. 



Habitat. In pools near low-water mark on the shore at 

 Musselburgh, Firth of Forth; not uncommon. 



Eemarks. This very slender Copepod does not resemble a 

 typical Mesochra^ but from its general form seems rather to 

 belong to that peculiar group repi'esented by Gylindropsyllus 

 and Leptopsijlhis. In the structure of its various appendages, 

 however, it is a true Mesochra. The first pair of swimming- 

 feet closely resemble those of Mesochra Lilljehorgii^ differing 

 only in the proportionally greater length of the end-joint of 

 the inner branches. The small clawed posterior foot-jaws 

 and the peculiar form of the fifth pair of thoracic feet are, 

 however, very good and distinct specific characters, inde- 

 pendent of the elongate and slender form of the animal. 



The species is named in compliment to Prof. W. C. 

 M'Intosh, the Scientific Director of the Fishery Board for 

 (Scotland. 



