212 



DK. W\ D. LAXG ON 



[vol. lxxiv, 



fusion of the proximal pair of apertural spines with the fused distal 

 prongs of the bifid apertural bar. The distal prong and the 

 proximal spine of each side lie nearly parallel and separated by a 

 -deep slot. The avicularia are reduced to a single pair placed one 

 ■on each side of the aperture immediately distal to the proximal 

 pair of apertural spines, directed obliquely towards the mid-line of 

 the aperture which they accompany, and carried up on a pedestal 

 to a level with the proximal rim of the secondary aperture. The 

 avicularia lie close alongside the apertural spines, are parallel with 

 them, and are separated from them only by a deep groove. There 

 are thus three structures lying alongside each other .and parallel 

 with each other : namely, the distal forks of the apertural bar, the 

 proximal apertural spines, and the avicularia ; also the first two are 

 separated by a slot-like hollow, and the last two by a deep groove. 

 The distal shield is lower than the proximal shield, and merely 



Fig. 7. — Diagram of the distal end of an cerium of a hypothetical 

 Primitive Kelestoma. Very much enlarged. 



Distal apertural spine. 



Aperture. 



Avicularium. 



Proximal apertural spine. 



• Distal fork of secondary aperture. 



Proximal fork of secondary 



aperture. 

 Costa. 



consists of a rim of secondaiy tissue continuous with the secondary 

 intercecial tissue lying between the aperture and the cecium distal 

 to it and containing large median lacunae. This distal shield over- 

 tops and obliterates the distal pair of apertural spines. Thus the 

 secondary apertural ring consists distally of a rim of secondary 

 tissue, proximally (somewhat laterally) of the circum-apertural 

 avicularia, then of the broken ends of the proximal pair of apertural 

 spines, and, finally, of the fusions of the proximal apertural spines 

 and the fused distal forks of the bifid apertural bar. Moreover, a 

 patch of secondary tissue with a more or less circular median lacuna 

 fills a gap in the ring between the apertures of the circum-apertural 

 avicularia and the broken ends of the proximal apertural spines. 

 The complete secondary aperture is diagrammatical^ shown in fig. 8 

 (p. 214), and an intermediate stage between this and the aper- 

 ture of the Primitive Kelestomine and occurring in a hypothetical 

 Primitive Kelestoma is shown in the accompanying figure (7). 



Evolution within Kelestoma appears to have affected colonial 

 habit, oecial size, costal number, and avicularian shape. The 

 asty progresses from incrusting, through erect and bilaminur 



