.218 DR. W. D. LANG ON [vol. lxxiv, 



VIII. The Ancestrcecium of Morphasmopora jukes-browsei 



(fig. 12, p. 217). 



It is fortunate that the ancestrceciimi of Morphasmopora juhes- 

 hrownei should have been preserved in one of the specimens of this 

 species in the British Museum collection (D 8005). It is to be 

 expected that its characters will fall into three categories : 



(1) primitive characters expressing remote ancestral influence ; 



(2) youthful characters exhibited in simplicity of structure due to 

 tender age ; and (3) advanced characters indicating high speciali- 

 zation. To the first cause is due the great number of the apertural 

 spines, the large size of the extraterminal front-wall, and, perhaps, 

 partly, the small cecial size and the small number of the costae ; 

 to the second, mainly, the small cecial size and, partly, the small 

 number of costae ; also the absence of secondary tissue ; while the 

 greatly enlarged proximal pair of apertural spines, their (presumed) 

 fusion with the apertural bar, and the bifid character of the last- 

 named structure are characters that fall under the third category. 

 It is interesting, in this connexion, to note that the ancestrcecium 

 of M. juJces-hrownei not only does not show any peculiar specific 

 •characters, but does not even proclaim the genus Morphasmopora. 

 The utmost systematic characters that it exhibits are those of 

 the subfamily Kelestominae. 



The ancestrcecium may be described as follows :- — The cecia are 

 about '3 mm. long, about *2 mm. wide, and oval in shape. The 

 extraterminal front- wall is very large, at least proximally. The 

 •costae are five in number. The apertural bar is bifid and very large, but 

 not so large, comparatively, as in the ephebastic cecia. The proximal 

 apertural spines are greatly enlarged, but they are thinner than 

 ^fche costae ; while the ephebastic apertural spines are about four 

 times as thick as the costae. Distal to the enlarged proximal 

 apertural spines are two pairs of long, thin, distal apertural spines. 

 There is no secondary tissue. 



IX. Summary. 



The Kelestominae are a subfamily of Pelma to poridae, characterized 

 by a bifid apertural bar. By stripping the diagnostic characters 

 from the various Pelma toporid subfamilies, a Ivvpothetical Primitive 

 Pelma toporid is obtained, from which the Kelestominae (as well as 

 the other subfamilies) can be derived. Similarly, a hypothetical 

 Primitive Kelestomine may be reconstructed, from which the two 

 Kelestomine genera (Kelestoma and Morphasmopora) can be inde- 

 pendently derived. Kelestoma is chiefly characterized by its great 

 -cecial length and the large number of its costae ; and evolution 

 within the genus is mainly concerned with an anabasis of colonial 

 -characters and cecial length, and a catabasis of costal number ; and 

 is expressed in the single lineage K. elongation — K. gradation — 

 K. scalare. 



Morphasmopora has two species, M. brgdonei, with four 



