part 3] THE KELESTOMIXJE. 211 



were all, the amount of calcium carbonate deposited in apertural 

 spines would be less in the Primitive Kelestomine than in the 

 Primitive Pelmatoporid from which it is derived ; and this is con- 

 trary to the fundamental principle postulated for cribrimorph 

 evolution. But the proximal pair of apertural spines is very 

 much thickened in the Kelestominae, and the loss of a pair is much 

 more than counterbalanced in this enlargement of the proximal 

 pair. It is difficult to see any positive reason why the spines 

 should be reduced in number ; but such a reduction is almost 

 universal in the Pelmatoporidae (the Casta noporinae provide ex- 

 ceptions;, and is shown by the ancestrcecia of many four-spined 

 forms having six apertural spines. In fact, the ancestrcecium of 

 Morphasmopora jukes-brownei (fig. 12, p. 217) has six. and the 

 proximal pair is already considerably enlarged. It seems, then, 

 that the reduction in the number of Pelmatoporid apertural spines 

 is a true catagenesis, and that the increased superfluity of 

 calcium carbonate released thereby must be laid down in other 

 parts of the skeleton. Because of parallel instances in other 

 subfamilies of Pemiatoporidae, the possession in Morphasmopora 

 brydonei of six apertural spines is regarded as a Primitive Pelmato- 

 porid rather than a Primitive Kelestomine character. 



To sum up : the Kelestominae are Pelmatoporid a? in which each 

 half of the apertural bar is bifid in a more or less oblique plane, 

 and each fork fuses with the corresponding fork of the other half ; 

 moreover, the fused distal forks fuse with the proximal pair of 

 apertural spines, which are very much enlarged. Other charac- 

 teristic, though not diagnostic features, are an abundant intercecial 

 secondary tissue with narrow, elongate lacunae, which tends to 

 spread into and fill up interstices in the rim of the secondary 

 aperture ; and one or two pairs of avicularia accompanying the 

 aperture of every normal cecmm. 



IV. Kelestoma Marsson (fig. 8, p. 214). 



And here Kelestoma and Morphasmopora part company. They 

 take opposite directions with regard to two correlated characters : 

 namely, comparative cecial length, and the number of costae. 

 Morphasmopora also carries to extremes certain modifications of 

 structure which Kelestoma does no more than initiate. In 

 Kelestoma, then, the oecial length greatly increases compared 

 with its width, from being less than twice as long as wide in the 

 Primitive Kelestomine to nearly three times as long as wide in 

 Kelestoma. The costa? also increase from less than ten in the 

 Primitive Kelestomine to over twenty in Kelestoma ; and the 

 pelmatidia have retreated somewhat from the mid-line, leaving a 

 broad median area of fusion. There is a considerable amount 

 of secondary intercecial tissue with elongate and fairly straight 

 median lacunae. 



The proximal shield of the secondary aperture is formed by the 



