G. M. Thomson. — On a new Species of Daphnia. 241 



The species described now is very similar in general form to the common 

 European D. pulex, and perhaps is only a variety of it. It differs however 

 in several structural points, and is therefore raised provisionally to specific 

 rank under the name Z>. similis. 



The following is a brief description of the animal : Valves oval in form, 

 not quite twice as long as broad, and produced posteriorly into a sharp 

 stout spine, which is about one-seventh of the length of the carapace in the 

 male, and about one-sixth in the female. The whole lower margins of the 

 valves, as well as the spine, are fringed with short pectinate hairs. 



The head in the male is nearly perpendicular in front, and ends in a 

 blunt beak, which bears the anterior antennse near its extremity : these are 

 1 -jointed, and bear a single filament, which is flexible and serrated at its 

 extremity. The whole length of these organs is nearly equal to the breadth 

 of the head. In the female the front of the head protrudes considerably, 

 and the beak is very acute. The anterior antennse are represented by two 

 very minute tubercles, destitute of setae, which are situated on the under- 

 side, close to the extremity. These organs appear to be in a more 

 rudimentary condition in our species than in any other hitherto described. 

 The posterior antennae are of normal form in both sexes, but are relatively 

 shorter than the same organ in D. pulex, being only a little more than half 

 as long as the carapace. All the joints are furnished with minute hairs. 



The posterior portion of the body is dorsally produced into four rounded 

 lobes, the upper one of which is the longest, while the lower bears two 

 long setose spines or filaments. The abdomen in the male is much rounded 

 at the anal orifice, and bears at that part on each side about 7 curved teeth, 

 which are somewhat distant from the 2 curved terminal hooks. In the 

 female the abdomen is slightly rounded on the lower margin, and bears 

 10-12 curved teeth close up to the terminal hooks. 



My specimens included one male, T ^th of an inch in length, and several 

 females, the largest of which were about -|th of an inch long. When living, 

 their colour, according to Mr. Chilton, was brick-red. 



DESCRIPTION OP PLATE XIII., Figs. 6-9. 

 Figs. 6-9. Daphnia similis. 



6. Adult female ; 7. rostrum of same ; 8. anterior antenna, male ; 9. extremity 

 of abdomen, male. 



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