A REVISION Ot" THE GENUS STEGANOFOUEI-LA. 267 



ous with the siibinarginal or tooth-bearing sclerite. In the 

 distal pai't of their course the space between the halves of 

 the main sclerite is Hoored by a transparent layer of cliitin 

 which has a sharp arch-like edge proximally (figs. 36, 41). 

 This space contains living tissue, as is shown by its behaviour 

 to staining reagents. It appears as a loop-shaped area, often 

 dilated distally, between the two parallel halves of the distal 

 region of the main sclerite. Each of these halves becomes 

 more and more prominent as it passes proximally, being pro- 

 duced on the proximal side of the occlusor tubercle into a 

 strong projection, which stands out to a considerable distance 

 beneath the upper surface of the operculum. The space 

 between this projecting edge and the more externally placed 

 base-line of the sclerite (on the proximal side of the occlusor 

 tubercle) often appears as a fenestra (cf. fig. 36), but is 

 more probably due to the fact that a thin vertical lamella of 

 chitin connects a broad base with a broad edge. There is 

 always a strong basal sclerite (h) ; and an oblique sclerite (o) 

 runs from the proximal projection outwards as an additional 

 buttress to this part of the operculum. 



The projecting ends are for the insertion of a strong 

 muscle, as I hope to show on a future occasion. This type of 

 operculum differs widely from the b opercula of the first 

 group of species, and I can hardly doubt that it is of great 

 functional importance to the colony. The possession of this 

 highly characteristic operculum may be regarded as indicat- 

 ing the relationship of the species to other species similarly 

 provided. The submarginal teeth of these b opercula are 

 nearly always strong. 



The median process of the b zooecia is usually obviously 

 narrower than that of the a zooecia. This is probably due to 

 the larger muscles of the b form, necessitating a wider 

 lateral recess, and a consequent compression of the median 

 process. 



Several very distinct types of A opercula occur in this 

 group of species. I begin with the species in which these 

 opo'cula are still of the ^' uudiffeientiati'd^' type. 



