A REVISION OP THK OENUS STEGANOPORELLA. 239 



not appeal' to vary to any great extent even in specimens 

 from widely different localities. In characterising the species 

 I have found tlie following features of special service : 



(i) Tiie opercula (a and b forms). 



(ii) The arrangement of the descending or distal part of 

 the cryptocyst. 



(iii) The form of the tube and of the median process. 



I have not derived any special assistance from the rosette- 

 plates, a character which, in the hands of Mr. Waters in 

 particular, has given valuable aid in the discrimination of 

 species. The rosette-plates of Steganoporella are large 

 and conspicuous. Their typical arrangement seems to be 

 identical with that described by Nitsche for Membrani- 

 pora meinbranacea.^ The zooecia are in longitudinal rows, 

 those of adjacent rows usually alternating, so as to give a 

 regular quincuncial disposition. Each zooecium typically 

 communicates with its neighbours by two proximal, two 

 distal, and four lateral rosette-plates on each side. It has 

 typically six neighbours, and it is connected with each one 

 by two rosette-plates, although there are many deviations 

 from this rule. I am unable to point out any constant cha- 

 racters by which the rosette-plates of one species differ from 

 those of another species, while in one and the same colony 

 vei'y different conditions may obtain in neighbouring zocecia. 

 As an instance of this I may mention the case of a fragment 

 of the encrusting form of S. neozelanica, in which the 

 proximal wall of four zooecia could be examined from the 

 inside. In one of these there were two large watch-glass 

 shaped ^ rosette-plates with their concavity opening into the 

 distal zooecium ; in a second there were two similar rosette- 

 plates with their convexity in the distal zocx3cium ; in a third 

 two flat rosette-plates were present ; and in the fourth none 

 could be discovered. 



In S. buskii, while four lateral rosette-plates may occur, 

 other zooecia may have only three, or even two. The proxi- 



' 'Zcitschr. f. wiss. Zool.,' xxi, 1871, p. 421 (fisj. 1). 



- Cf. Waters, ' Clialletiger Reports,' pt. Ixxix, 1888, |). 13. 



