F. D. Longe — Oolitic Polyzoa. 31 



less to the whole class, and capable of being developed in some 

 form or other by any species, than to regard such a mere appendi- 

 culate organ as the exclusive property of one primary division of 

 the class. 



In a specimen of D. patina which I have before me, the difference 

 in the shape of the closed cells and those with open orifices is very 

 marked.^ The latter are cylindrical tubes which rise free from the 

 coeuoecium at right angles to the surface from which they spring ; 

 the former are decumbent, having their upper parts only slightly 

 elevated above the surface of the coenoecium in which the rest of the 

 cell is immersed. The orifices of the protuberant cells are simply 

 the round extremities of the tubes ; the orifices of the decumbent 

 cells are oval or elliptical. 



The difference in the shape of the orifices in the two kinds of cells 

 is evidently due to the difference in the direction of their growth. 

 If a conical tube is truncated, at right angles to its axis, the section 

 made is round ; if it is truncated in an}'- other angle, the section 

 made is more or less elliptical, according to the angle made by the 

 plane of truncation with the axis of the tube. This principle would 

 seem to afford a sufficient explanation of the difference in the shape 

 of the orifices in the erect cells, and of those in the decumbent or 

 semi-erect cells, in D. patina, and to be more or less illustrated by 

 the variation in the shape of the orifice in many other coenoecia, 

 belonging both to Cyclostomatous and Cheilostomatous forms. 



But the difference in the shape of the erect and the decumbent 

 cells in B. patina is the recognized distinction between the tubular 

 shape of the Cyclostomatous cell and the oval shape of the Cheilo- 

 stomatous cell ; and if the appearance of these two types of cell form 

 in the same coenoecium is considered in connexion with the evidence 

 which the Oolitic Escharoids furnish, first of a similar combination 

 of these different cell forms in the same lamella, and secondly of 

 the gradual disappeai'ance of the protuberant cells, and the develop- 

 ment of the typical cell features of the Cheilostomata in the same 

 group of forms, the problem which puzzled the authorities to whom 

 I have referred above in dealing with these forms is solved. The 

 decumbent cells in Diastopora may be regarded as ancestral 

 Cheilostomatous cells, and Diastopora itself as the parent stock from 

 which many, if not all, of the families of the Chalk and subsequent 

 periods, grouped as Cheilostomata, have been derived. 



In speaking of the oval shape as typical of the Cheilostomatous 

 cell, I am fully borne out by Professor Busk's description of the two 

 types in his introduction to the Crag Polyzoa. It is, however, im- 

 portant to observe that Johnston defines the cell of the Celliporina 

 (the group which in his system embraces the greater portion of the 

 Clieilostomata of later systematists) as being " oblong or oviform." 

 He evidently introduced the term "oblong" to meet the case of the 

 Escharidce. The principal representative of this group among living 

 forms is unquestionably ii^sc/mra/o/mcea, and it is clear that the cells 

 in this form are rather fusiform and oblong than oval. The cells are 



1 See Fio-iu-e 8. 



