150 SIDNEY F. HARMEE. 



that the part which was described as " tentacle-sheath " in the 

 ovicell of Crisia corresponds with the thin-walled embryo- 

 phore of Stage E of Tubulipora (fig. 25), whatever may be 

 the morphological character of this space. 



The separation of the genera of Cyclostomes is notoriously 

 difficult. In Crisia, Tubulipora, and Lichenopora we 

 have three extreme cases, which there can be no difficulty in 

 recognising as distinct. The fact that their ovicells belong to 

 three entirely different types renders the ultimate definition of 

 the genera of recent Cyclostomes a more hopeful task than it is 

 Sometimes supposed to be.-^ Crisia may be characterised as a 

 genus in which the ovicells are modified zooecia dilated into a 

 pear-like form, the region of the oceciostome, as in other 

 Cyclostomes, not sharing in this dilatation. The ovicell is 

 from the first an ovicell ; and although its morphology is 

 indicated by the appearance of a polypide-bud, the bud never 

 becomes a functional polypide. 



In Tubulipora the dilatation of the ovicell is usually 

 much more marked, and the ovicell is commonly lobed. Its 

 lobes are developed owing to the formation of zooecia distally 

 to the ovicell, and the latter is accordingly obliged to divide 

 into two portions which grow round the sides of the zooecium 

 or series of zooecia. The degree of lobing corresponds with the 

 extent to which young zooecia are developed distally to the 

 ovicell. Two lobes may become contiguous on the distal side 

 of a zooecium, but probably do not fuse ; and the embryophore 

 also is composed of a series of divaricated lobes which do not 

 unite (in the species examined). The young ovicell differs 

 from that of Crisia in beginning life as an ordinary zooecium; 

 and the duration of this period is accurately indicated by the 

 extent of the proximal cylindrical (or pyramidal) portion of 

 the ovicell. 



In Lichenopora, as in Tubulipora, the ovicell is at first 



a zooecium, but has more than one functional polypide (in L. 



verrucaria). The principal characteristic of the ovicell is, 



however, the mode of growth by the addition of peripheral 



1 Cf. Gregory, No. 12, p. 21. 



