220 ABYLA LEUCKARTII. 



distinguishing the species from the Eudoxid described by Chun as C. sagittata. 

 Even in the smallest examples it already shows the characteristic form and 

 asymmetry. Its various angles and facets have been described by Lens and 

 Van Riemsdijk; but their account is so complex that a more simple summarj', 

 with the accompanj'ing figures, may be useful for identification. The bract 

 is composed of five distinct facets (six if we follow Lens and Van Riemsdijk in 

 applying this term to the hydroecial cavity). These are a ventral, an apical, 

 and three which together compose the arched dorsal surface. The apical 

 (anterior) is an irregular tetrahedron, placed obliquely. The three dorsal 

 facets are separated from each other by two dorsal ridges, running to the base 

 from the two dorsal angles of the apical facet. These ridges are not symmetri- 

 cally placed, for the right hand one lies nearly in the main longitudinal axis 

 of the nectophore, whereas the left hand one is far to the left of it (Plate 15, 

 fig. 4). Consequent on the positions of the ridges the three dorsal facets are 

 asymmetrical, the left hand one very short, the central and right hand ones 

 much longer and broader. Lens and Van Riemsdijk suggest that the asym- 

 metry is due to the absence of a ridge which may be supposed to have primitively 

 been present on the right hand side, subdividing the right dorsal facet longi- 

 tudinally. But it seems to me that no such assumption is required. If we 

 compare the bract of this form with that of Abyla trigona ("Amphiroa alata") 

 we find that its apical facet corresponds in position to that of the latter, and 

 that it would require only a slight distortion to make its mid-dorsal facet corre- 

 spond to the dorsal, its right and left dorsal facets to the right and left laterals 

 of "Amphiroa." In other words were the apical facet of Ceratocymba a true 

 rectangle as it is in Amphiroa, instead of a rhomboid with unequal angles, and 

 were the basal outline transversely truncate instead of angular, the asym- 

 metry characteristic of the former would disappear, and the two would agree. 

 If we accept this explanation it is easy to derive the form of the adult bract from 

 that of the younger one described above. The two laterals of the latter would 

 correspond to the right and left dorsals, the dorsal to the mid-dorsal of the 

 former. The only facet of the "Amphiroa" which is not represented in 

 "Ceratocymba" is the basal one. This is suppressed by the form and large size 

 of the ' hydroecial cavity. A diagnostic character only second in importance 

 to the asymmetry of the bract is afforded by the somatocyst, the stout single 

 median descending branch of which is curved dorsad near its tip in an extremely 

 characteristic manner. So far as I know, no other Amphiroa-like Eudoxid 

 possess this character; and it therefore assumes greater systematic importance 



