10 Vniversity of California Publications in Zoology. [Vol. 4 



still larger clear part ; situated on the right side of the body, close 

 to the posterior end of the endostyle. Gonads, one on each side 

 of the body, that on the left in front of the intestinal loop (pi. 1, 

 fig. 7). 



This species, clear-cut in its generic characters and also 

 sharply set off from any other species of the genus, needs little of 

 comment beyond what is brought out by the diagnosis. While it 

 is one of those species so welcome to the systematist because of the 

 positiveness in both its geueric traits and in its specific differenti- 

 ation from other known ]\Iolgulas, it is at the same time a good 

 example of those species which present such a combination of spe- 

 cific characters as to make the question of what its closest affinities 

 within the genus are, so difficult, but at the same time so interest- 

 ing. For example, as regards surface covering, form, hypophysis 

 mouth, number of folds in the branchial sac, and position of left 

 gonad, M. regularis has much in common with M. tenax Traustedt, 

 and on the whole one may conclude that it is more closely related 

 to this than to any other species. At the same time, as regards the 

 mantel muscles, the form of the gonads and the structure of the 

 so-called liver, the two differ sharply. M. tenax has no such de- 

 velopment and disposition of mantle muscles about the siphons 

 as M. regularis presents us with (pi. 1, fig. 7). The testis of tenax. 

 as shown by Hartmeyer '03, is a rounded single mass on each side 

 instead of a whole series of small elongate lobes as in our species ; 

 and is situated to one side of the ovary instead of being scattered 

 round it so as to give the latter a central position with reference 

 to it. The liver in tenax is, according to Hartmeyer, ' ' very excep- 

 tional in its enormous development." Nothing of this sort is 

 found in M. regularis. But M. arctica Kiaer is a species in which 

 the mantle musculature is very similar to that of M. regularis. 

 Likewise the structure and relation of the male and female 

 gonads in this far northern species are much like those of 

 M. regularis. In the latter the relatively great elongation of the 

 combined male and female glands, with the testes divided up into 

 distinct and quite widely separated lobes, these being deployed 

 around the ovary and scattered along its whole length, is to be 

 regarded as an extreme expression of the same type as that repre- 

 sented by the glands of M. arctica. In this latter species, however, 



