802 SIDNIiY F. HAEMKR. 



on tlic proximal side of tlio condyles. It appears to me that 

 the function of the tongue is partly to close the aperture of 

 the compensation-saCj and partly to give suitable leverage 

 for the action of the divaricator muscles. 



S. linearis is characterised by the possession of a pair of 

 sul)oral avicularia, with pointed mandibles. Hincks (1880, 

 p. 251) has described in this species a form of " ooecium/' 

 associated with a rudimentary zooecium. My preparations 

 show clearly that these supposed ovicells (fig. 48, nvic.) are 

 in reiilit}^ gigantic suboral aviculariii/ of the tj'pe well known 

 in Retepora monilifera, var. munita, Hincks." The 

 greatly swollen region distal to the mandible is occupied l)y 

 the enormous occlusor muscles, which differ only in size from 

 those of the ordinary avicularia, their relation to wliich is 

 further shown by the fact that a single gigantic avicularium 

 may lie obliquely across the zocecium instead of longitudinally 

 as in fig. 48. The compensation-sac (fig. 52,c.s.) of the 

 zoQScia is small, and even in zocecia with fully matui-e pol}'- 

 ]ndes is commonly no larger than in the specimen figured. 



The sinus of the operculum (fig. 52) fits into a correspond- 

 ing emargination in the frontal shield. At a deeper level 

 are seen the two condyles (cond.) which constitute the hinge. 

 The frontal wall is of considerable thickness, and its distal 

 margin, in the neighbourhood of the sinus, is bevelled off in 

 precisely the way noticed in Euthyris clathrata (fig. 26). 

 This results in the formation of a groove, the outer end of 

 which is closed by the sinus of the operculnm, while the inner 

 end (fig. 52, x) is distinctly larger. When the operculum is 

 open, its plane must be a])proximately parallel to the part of the 

 bevelled surface which lies in the middle line. A short tube 

 is thereby formed by which water can enter or leave the 

 compensation- sac. 



The compensation-sac develops as in Lejir.-ilia palla- 

 siana. The calcification takes place beneath tlio epitheca, 



> Waters (1S85, p. 0, and 1892, p. 271) lias previously slated that lliis is 

 the case. 



2 'Ann. Mag. Nat. Hist.' (5), i, 187S, p. Sfil. 



