234 SIDNEY F. HARMER. 



by fine calcareous processes, giving it a fimbriated appear- 

 ance. 



In Steganoporella evidence of asymmetry of the tube 

 is conspicuous in certain species, as in S. lateralis (fig. 1), 

 where the condition hardly differs from that in Siphono- 

 porella, and in S. magnilabris (fig. 10), where the 

 asymmetry, though less apparent at first sight, is equally 

 striking in reality. 



In other species of this genus the tube tends to become a 

 median structure ; and its appearance in most cases is much 

 modified by the outgrowth of lateral calcareous flanges from 

 its upper wall. These usually take the remarkable form seen 

 in figs. 3, 8, etc. In most species the roof of the tube can be 

 distinctly seen as a convex floor (fig*. 10) to the cavity which 

 is formed by the development of these flanges. It is not 

 easy to suggest a satisfactory explanation of this structure. 

 The lateral flanges have the effect of more completely de- 

 limiting the lateral or muscular recesses. They also strengthen 

 the distal edge of the median process which corresponds 

 with the base-line of the operculum. I cannot find evidence 

 that the cavity of the median process contains any organ of 

 importance ; and I think it not improbable that it prevents 

 the too forcible retraction of the epitheca by the contraction 

 of the depressor muscles. If the epitheca should be de- 

 pressed so far as to come into contact with the edges of the 

 cavity in question, the resistance of the fluid in the cavity 

 (which is quite closed basally) would probably act as a 

 cushion, which would be less likely to tear the epitheca than 

 would a sharp calcareous edge unaided by a fluid cushion. 



The distal part of the cryptocyst has hitherto been spoken 

 of as if it descended equally both medianly and laterally. 

 Although this may be approximately true of some species, 

 as of S. Buskii (fig. 13), the arrangement is usually compli- 

 cated by the fact that the part which descends into the 

 cavity of the median process has a different slope from that 

 part which forms the floor of the lateral recesses. 



One of the most striking features of the genus Stegano- 



