A TlEVISrON OP THR OENUS STRGANOPORELLA. 235 



p or el la is the dimorpliism of tlie zooecia, which usually, 

 thoug-h apparently not always, occurs. The dimorpliism 

 is due not only to differences in the form and size of the 

 calcareous parts of the zocecia, but in an even more striking 

 way to differences in the opercula. These, in Steg-ano- 

 porella, are always strengthened by vertical bars of 

 chitinous substance, which stand out from their lower side 

 like the joists from the lower part of a floor. In the one 

 form of opercula (which, with their corresponding zooecia, I 

 shall designate as '^a") the operculum is usually semicircular, 

 with the main strengthening girder or bar concentric with 

 its curved margin (fig. 19). These zooecia may be regarded 

 as the equivalents of the ordinary zooecia of the Cheilostomes. 

 In the " B " form the operculum is usually a good deal larger, 

 and has an entirely different form of strengthening bar, 

 which may be roughly A-shaped (fig. 41) or D-shaped (fig. 14). 

 The A and b forms of opercula appear to differ in their mus- 

 cular system, as I hope to show on a future occasion. A more 

 obvious distinction depends on the character of the chitin- 

 ous teeth borne by the opercula. While a opercula may be 

 quite toothless (fig. 19), or with two (fig. 15) or four (fig. 24) 

 strong teeth on the main thickening bar, or in one case (S. 

 magnilabris, figs. 45, 46) with numerous small submarginal 

 teeth, the b opercula are nearly always characterised by the 

 greater or smaller development of submarginal teeth (figs. 

 36, 38). In one case (S. alveolata, fig. 41) each of these 

 teeth fits when closed into a socket in the calcareous oral 

 shelf of the zooecium (fig. 12). I have found the opercula 

 eminently serviceable in the discrimination of the species. 



The nature of the dimorphism of Steganoporella has 

 formed the subject of some discussion, though the sugges- 

 tions that have been made are mere guesses. Hiucks and 

 Busk have regarded the epi- cryptocystal chamber as ooecial 

 in function, in the b form at least; although Jullien^ has 

 rejoined that part of the cavity in question contains 

 muscles, and cannot be regarded as an internal ovicell. This 

 I Loc. cit. (1888). 



