258 DR J. H. ASHWOHTH AND DR JAMES RITCHIE ON THE 



1906, and of two slides containing a considerable number of sporosacs of both 

 sexes, from the same source, mounted in formalin. The eruption of Vesuvius of 

 that year had just concluded, and the showers bf fine dust had produced great 

 mortality among the marine organisms. To this cause we attribute the fact that, 

 of the seven shells, one bore a dead colony, and five others bore colonies from which 

 the ordinary polyps had almost wholly disappeared. In these five colonies, however, 

 living substance in the form of coenosarc in the stolon was present, and a number 

 of blastostyles — though on two of the colonies there were very few — had managed 

 to survive. The remaining shell bore a colony in good condition, and it is on this 

 that most of our observations have been made. 



General Description of the Colonies of Dicoryne conybearei.* 



(See PI. VI.) 



The colonies are in each case on small, gastropod shells of the genus Nassa, and 

 those which are well established spread over the entire shell, showing no preference 

 for any particular portion, though, as a rule, the individuals in the neighbourhood 

 of the mouth of the shell are more luxuriantly developed. 



In young examples the stolon can be easily seen forming an irregular meshwork 

 on the surface of the shell, but in older examples the interspaces between the strands 

 of the stolon become filled with diatoms and debris so that a uniform rusty-grey 

 covering is formed, obscuring the stolon and obliterating the finer marking of 

 the shell. 



From the stolon, at irregular intervals, arises either a short stalk terminated by 

 a single hydranth, or a longer branched stem bearing two, three, or four, (rarely 

 more) hydranths. The stems are rusty grey in colour, due, as in the case of the 

 stolons, to a covering of foreign bodies (in this case chiefly volcanic dust) adherent 

 to the chitin. The perisarc of the stem is irregularly corrugated or wrinkled, and 

 at its distal end widens slightly, forming a small, transversely wrinkled cup, which 

 bears a dense coating of foreign particles. This elementary hydrotheca covers, in 

 the full-grown polyps, only a very small proximal portion of the hydranth body. 

 The hydranths are ovoid or fusiform according to the state of expansion. Each has 

 a prominent conical hypostome surrounded by a whorl of filiform tentacles. The 

 number of tentacles varies from six in the youngest specimens observed to sixteen 

 in the largest adults. 



The sporosacs are borne on blastostyles which arise either directly from the 

 stolon or from the stems of polyps. The blastostyle varies in form according to 

 its phase of development. It is at first almost cylindrical or slightly dilated at 

 its distal end, which is armed with numerous nematocysts, but soon becomes 

 swollen about the middle of its length and is henceforward more or less vase-shaped. 



* For the diagnostic characters of this species see p. 282. 



