206 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



made the basis for generic separation since it is merely a case of permanence in 

 the Atlantic species of a character retained up to a very late stage in growth by 

 the Pacific form ; and therefore a modification of my earlier (1909) generic charac- 

 terization is necessary. In the condition of the canals the specimens are much 

 more closely related to S. simulans than to S- geometrica, but they are separated 

 from both not only by the permanently blind centripetal canals, but also by other 

 characters of sufficient importance to warrant the institution of a new species. 



Sibogita nauarchus, sp. nor. 



Fig. 1-8. 



Lat. 39° 49' ft. ; long. 70° 16' W. ; 175 fathoms to surface, 6 specimens. 



Lat. 39° 39' N. ; long. 71° 48' W. ; 150 fathoms to surface, 1 specimen. 



The specimens all have well developed gonads, in which the sexual products 

 are apparently mature. 



In general external appearance S. nauarchus closely resembles S. simulans. 

 The specimens are, however, larger than the three recorded individuals of the 

 latter, the largest in the present series measuring 37 mm. in height by 40 mm. in 

 diameter, as against 30 and 22 mm. respectively for the "Albatross" specimens 

 of S. simulans. The single known specimen of S. geometrica is described by 

 Maas 1 as being 38 mm. high by 30 mm. in greatest diameter. The general out- 

 line of the bell is rounded ; the gelatinous substance fairly thick and stiff. An 

 important feature of S. nauarchus is the presence of a deep funnel-shaped apical 

 depression, clearly shown in the type (Fig. 1). This character must be regarded 

 as normal, since it is well developed in all six specimens ; and since no correspond- 

 ing structure occurs either in S. geometrica or in S. simulans it is no doubt of 

 specific significance. 



Both Maas and myself have already recorded a lateral flattening of the bell in 

 Sibogita, whereby one diameter is much greater than the other ; and since such a 

 flattening was observed in all the specimens of S. nauarchus before preservation 

 it is probably normal. However, the plane in which the flattening occurs is not 

 invariable, it being either radial or interradial. Thus, while in both the Eastern 

 Pacific specimens of S. simulans, even before preservation, there was strong radial 

 flattening (Bigelow, loc. cit., pi. 5, fig. 5), in the present series the flatten- 

 ing is radial in two specimens, and interradial in 4. Prom this evidence it ap- 

 pears that the flattening is not a structural feature, but a contraction-phase which 

 is readily assumed. 



Manubrium- The manubrium, as in both previously known members of the 

 genus, is barrel-shaped, about two-thirds as broad as long, and about one-half as 

 long as the bell cavity is deep (Fig. 1), and it is readily distinguishable into basal, 

 gastric, and labial portions. As in S. simulans it is cruciform basally (Figs. 2, 4). 



Canal system and tentacles. The collection contains an interesting series of 



1 Die Craspedoten Medusen der Siboga-Expedition. Ukkom. op. Zool. Bot., 

 Oceanogr. en Geol. Gebied. Siboga-Expeditie. Monogr. 10, p. 17. 1905. 



