210 DR. W. D. LANG ON [vol. lxxi\> 



aperture. There is always, however, a deep cleft between these 

 pedestalled avicularia and the proximal apertural spines, that may, 

 nevertheless, be imperfectly filled with secondary tissue — imper- 

 fectly because of the median lacuna in the secondary tissue. The 

 avicularia are always more or less distally directed. 



It is difficult to account for the bifid splitting of the apertural 

 bar. Only the proximal fork of each side bears a pelmatidium, 

 showing that each half is not formed of two adjacent costae fused 

 proximally and diverging distally. The Kelestomine apertural bar 

 is to some extent comparable with that type, occurring indepen- 

 dently in more than one family of cribrimorphs, which has a median 

 spine that (like the fused distal forks of the Kelestomine apertural 

 bar) fuses with the proximal pair of apertural spines to form the 

 proximal shield of a secondary aperture. In such cases (fig. 6) 



Fig. 6. — Diagram of the distal end of the oecium of a Pelma- 

 toporid having a median process of the aperturat bar fused ta 

 the proximal pair of aperturat spines. Yery much enlarged. 



Distal apertural spine. 



Aperture. 



" Proximal apertural spine. 

 Median process of apertural bar. 



Ayieularium. 



Apertural bar. 



Pelmatidium. 



Costa. 



it is often obvious that the median spine is double : that is, formed 

 of two fused processes, one from each half of the bar ; and often, 

 in the Pelmatoporidse, the pelmatidium- or pelma-bearing end of 

 each half of the bar is turned somewhat towards the proximal end 

 of the oecium : that is, in a direction opposite to that in Avhich the 

 median process is turned. This gives a tendency to the formation 

 of a diamond-shaped depression in the middle of the apertural bar. 

 Now the Kelestomine apertural bar has a similarly diamond-shaped 

 opening between its two pairs of fused forks, formed, apparently, 

 in a similar wa}^. Thus it may be regarded as a further develop- 

 ment of the apertural bar with a median spine ; or the bar with a 

 median spine a further fusion of the Kelestomine bifid bar. The 

 former is the more probable ; since the spined structure occurs 

 several times in different families and genera of cribrimorphs and 

 in most cases can be derived from a simple apertural bar. 



One other character of the Primitive Kelestomine must be 

 noticed. The apertural spines are reduced to four in number from 

 the six of the Primitive Pelmatoporid. It is obvious that, if this 



