60 DE. V. H. CARPENTER ON SOME ARCTIC COMATUL^. 



the same size the joints are as wide or wider tlian long, and 

 present the shape characteristic of the adult individual. The dif- 

 ference is so marked, that Fischer can hardly have overlooked it, 

 if he really did compare the two types ; and it indicates very 

 clearly that A. prolixa and A. tenella are by no means so closely 

 identical as lie supposes. 



5. The length o£ the syzygial intervals throughout the arms is 

 also very different in the two forms. I have carefully examined 

 a large number of arms of A. tenella from many different loca- 

 lities ; and I find that after the third syzygy, which is almost 

 invariably in the twelfth brachial, the syzygial interval is nor- 

 mally only two joints. The second interval may sometimes be 

 two instead of three, and more rarely four ; so that the third 

 syzygy comes on the 11th or 13th, instead of on the 12th 

 brachial ; but the remaining syzygies occur almost invariably on 

 every third joint. The interval is sometimes, though rarely, in- 

 creased to three joints ; but I have never found it rising to four 

 or five, as mentioned in Miiller's description of the type*. In 

 three of the longest unbroken arms which I have met with the 

 succession of intervals was — 



22222 2 222222222 or 2'\ 



22 2 222222232222 or 2'°, 3, 2\ 



2223 2 2222222222 or 2^3, 2". 



I have also examined twenty specimens of A. prolixa from 

 seven different localities, and have recorded the sequence of the 

 syzygies in the five longest arms remaining to each. I find that 

 the syzygial interval is generally 3 joints, but may sometimes be 

 4, 5, or 6 ; so that the third syzygy may come anywhere from the 

 12th to the 15th brachial. The following intervals may be of 

 1-G joints, 3 being by far the most frequent, sometimes occurring 

 sixteen times in succession, though intervals of 2 joints are not 

 uncommon, especially in the first third of the arms. The fol- 

 lowing Table shows the distribution of the syzygies in the different 

 groups of individuals which I have examined. 



* " TJeber die Gattung ComaUila, Lam.,und ihre Arten," Abh.indl. d. k. Akad. 

 d. Wiss. Berlin [1847], 1849, p. 254. 



