208 



DK. W. D. LANG OX 



[vol. lxxiv, 



On comparing, now, the Primitive Kelestomine with the Primi- 

 tive Pelmatoporid, the former will be seen to have advanced upon 

 the latter in the following particulars: — the cecium is larger; there 

 is present some secondary intercecial tissue ; the apertural bar is 

 complicated ; the apertural spines are reduced in number, and the 

 proximal pair are differentiated in connexion with the apertural 

 bar ; the aperture is more advanced in shape ; and the avicularia 



are definite in numbers and 

 Fig. 3. — Diagram of an oscium and 



portions of six others, very much 



enlarged, to show how secondary 



tissue arises as contour-like 



ridges and buttress-like spurs in 



the intercecial valleys. (Secon- 

 dary tissue is the thick black 



line.) 



position. The diagnostic Ke- 

 lestomine character is the 

 bifid apertural bar. The other 

 modifications, even the en- 

 largement (though in a less 

 degree) and fusion with the 

 apertural bar of the proxima 1 

 pair of apertural spines, are 

 common developments, or 

 occasionally seen in other 

 Pelmatoporid subfamilies. 

 Nevertheless, the develop- 

 ment of secondary intercecial 

 tissue and the behaviour of 

 the avicularia must also be 

 further examined, if the- 

 complexities of Horphas/no- 

 p>ora 'jukes -brownei are to be 

 fully understood. 



The development of secon- 

 dary intercecial tissue may 

 be considered first. In the 

 Pelmatopondae generally, 

 secondary intercecial tissue 

 first appears as a contour- 

 like ridge running round the 

 front- wall in the neighbour- 

 hood of the termen (fig. 3) 

 and occasionally sending out 

 buttress-like spurs that run 

 across the intercecial valleys 

 and meet the ridge of secondary intercecial tissue of the opposite 

 cecium. As these contour-like ridges and buttress-like spurs are 

 reinforced by more calcareous tissue, it appears as if the inter- 

 cecial valleys were filled up to the levels of the termina with 

 intercecial secondary tissue, but that this tissue had median lacuna? 

 (elongate between two cecia, but triangular where three cecia meet) 

 at the bottom of which the extraterminal front- wall can still be- 

 seen (fig. 4). Next, these lacunae become smaller, their floors 

 become covered Avith secondary tissue, and, finally, they become 

 obliterated, while the intercecial valleys become so full of secondary 

 tissue that this tends to overflow the intraterminal front-wall 



