302 



the distal wall is provided with an expansion ending in a thickened, 

 crenulated margin, which partly separates the ocecium from the zo- 



oecium Sderodomus n. g. 



(Bifaxaria Busk, p. p.) 

 The peristome is tube-shaped, projecting, without avicularia, pro- 

 vided with a median opening; in the ocecium-bearing zocecia there is 



no expansion of the distal wall Tessaradoma Norman 



(Porina d'Orb., p. p.) 



S. denticulatus Busk. 

 Bifaxaria denticulata Busk, Challenger, Zoology, Vol. X, Part I, 1884, 



p. 82, PL XXIV, fig. 3. 

 Bifaxaria denticulata Waters, Challenger, Zoology, Vol. XXXI, Part III, 



1888, p. 15, PI. II, fig. 31. 

 — — Waters, Exped. Antarctic Beige, Bi-yozoa, 1904, 



p. 59, PI. VIII, figs. 14 a, b. 

 (PL XIX, figs. 18 a-18 c, PI. XXII, fig. 14 a). 

 The zooecia indistinctly separated, elongated, thick-walled, strongly arched, 

 increasing evenly in width from the narrower, proximal end and obliquely ascend- 

 ing towards the secondary, terminal aperture, which at a certain age forms almost 

 a right angle with the proximal part of the distal zooecium. They consist of an 

 extremely hard and solid, finely striated calcareous mass, which is provided with 

 as a rule fairly densely placed, round or oval, scattered pores leading into long, 

 more or less curved canals. As the colony gradually increases in thickness these 

 pores come to be situated at the bottom of narrow, channel-like concavities, which 

 increase considerably in length with age ^nd give the surface of the colony a 

 characteristic, grooved or longitudinally furrowed appearance. In quite young zooe- 

 cia the poi'es may even sometimes be extremely rare in a median belt along the 

 frontal wall. 



I have not been able to determine the form of the primary aperture, nor have 

 I been able to find any operculum. In the youngest, undamaged zooecia I have 

 been able to find, there is a secondary, more or less regular, broad but low, 

 semicircular aperture, within the proximal margin of which there is a low, but 

 broad, oblique tooth-like projection (PL XXII, fig. 14 a), which on the one side 

 grades into the lateral margin of the aperture and becomes gradually higher 

 towards the other side, where it ends in a rounded, rectangular or obtuse-angled 

 edge not far from the lateral margin. With the exception of quite few zooecia, in 

 which an outer, peristomial avicularium is wanting, the peristome in the younger 



