A REVISION OF THE GENUS STKGANOPORELLA. 283 



wall is the line of insertion of the cry ptocyst, forming the proxi- 

 mal boundary of the coi-responding lateral recess. The thin 

 line running transversely from this to the opposite lateral wall 

 may or may not be present. If it is absent, the tube is com- 

 pleted by the basal wall, and we have hei'e a case precisely 

 like that of Siphonoporella delicatissima (see p. 232), 

 where the tube rises asymmetrically from one side of the 

 base of the zooecium. If the thin line is present the tube 

 has a completed wall, and is connected with the basal wall 

 by a thin transverse septum, which is part of the cryptocyst. 

 If the cryptocyst of both lateral recesses meets the basal 

 wall two thick curved lines appear. 



The asymmetry of S. magnilabris, taken in conjunction 

 with what has been said under S. lateralis, seems to me 

 to have an important bearing in showing the derivation of 

 Steganoporella from an asymmetrical ancestor. 



The lateral walls of the tube are well developed. The 

 lateral recesses are often partly roofed in by a thin flange of 

 calcareous uiatter more or less continuous with the sides of 

 the median process, but principally developed at the outer 

 side of the lateral recess. 



The epithecal sclerites are short, not very conspicuous, and 

 they are widely separated from the operculum, b zocBcia 

 occur in large numbers. In some parts each b is followed 

 in the longitudinal row by two a, and the second of these 

 by another b ; in others, three or more a intervene between 

 each two b, and sometimes only one a. 



The b opercula (fig. 44) vary a good deal in size and shape, 

 without, however, losing their distinctive characters, the most 

 important of which is the arrangement of the teeth. These 

 are developed along the whole length of the submarginal 

 sclerite ; the proximal ones are small, but the distal ones are 

 very long, strong and recurved. Minute teeth may excep- 

 tionally occur between the larger ones (as in the figured 

 specimen). The parallel part of the main sclerite is usually 

 moderately long, though not so long and narrow as in S. 

 Haddoni. The b operculum is commonly elongated, with 



