Oscar Sund. [Xr. 6 



In the diagram (fig. 4) I have indicated with black spots the 

 egg-bearing individuals, and with rings the eggless individuals, of 

 each of the three species in question, the measure used being the 

 length of carapace from hinder edge of ocular sinus to edge of 

 earapace along a horizontal line (C). For the sake of com- 

 parison with Kemp's figures however the data upon the diagrams, 

 fig. 2 and 3 are based upon measurements of the length (Cr) of the 

 carapace along the median line. I have chosen,' and use, the 

 former measurement when dealing with all sorts of "macrura", 

 because it is not liable to errors due to different degrees of con- 

 traction or inflexion of the abdomen, and because it may always be 

 certainly and easily determined by means of a pair of compasses. 

 Neither is its accuracy affected by the very variable length of the 

 rostrum. Altogether I think that this measure much better repre- 

 sents the real size of a macrurous crustacean, than does the old 

 „from tip of rostrum to tip of telson". AVhen working with lob- 

 sters I found that the weight of the animals corresponded more 

 nearly with the dimension C than with the entire length, and of 

 course, the determination of the latter is more liable to errors than 

 the former. 



Stephensen (1912) arrives at the same negative conclusion as 

 I do regarding the distinction between P. midtidentata and P. 

 princijKilis on the basis of the comparative measurements (though 

 the specimens which he measured as P. princixKtlis were in fact 

 P. tar da!), but on the other hand he has discovered a new cha- 

 racteristic, viz. the carina on the sixth abdominal somite, which he 

 describes and figures (p. 68). Still he maintains that there is only 

 one species. I can only agree with him upon the validity of this 

 new character, and I think it must be admitted that the set of 

 characters enumerated below will suffice for the separation of the 

 two species: 



1. The form of the rostrum (Kemp, 1910). 



2. The form of the antennal scale (Kemp, 1910). 



3. The armament of the base of the 2nd periopod (Kemp, 1910). 



4. The carinaon the 6th abdominal segment (Stephensen, 1912). 



5. The form of the outer urapod (se fig. 7). 



6. The form of the telson (Kemp, 1910). 



7. The colour. 



Regarding the character derived from the number of spines 

 on the basis of the 2nd pereiopod, Stephensen (p. 68) argues that 



