THE MOKPHOLOGY OF THE CHEU.OSTOMATA. 829 



tliere is evidence that the frontal shield is formed by a 

 series of horizontal lobes developed from the main spines. 

 These latter are indicated with great distinctness as a curved 

 series of conical calcareous papilke, the truncated, end of 

 which is a membranous fenestra, and stands up at a higher 

 level than the frontal shield, which is encircled by the 

 papilla). The radiating series of pores here correspond with 

 the intervals between the spines, as in C. philomela. 



The existence of great differences between the opercula of 

 different species at present referred to Cribrilina suggests 

 that the genus is an unnatural one, representing a stage in 

 the evokition of the Lepralioid. zooscium, which has been 

 arrived, at independently in several cases, and is merely the 

 result of incomplete lateral fusion of the bars which compose 

 the frontal shield. 



VI. Classification of thp] Cheilostomata. 



While recognising the danger of drawing taxonomic con- 

 clusions from the study of a single organ, I cannot but think 

 (with Jullien) that the consideration of the frontal surface is 

 capable of settling some questions of this kind in the Cheilo- 

 stomata. It would, I think, be premature to propose a new 

 classification, but the observations above described suggest 

 certain lines along which the existing classifications may be 

 amended. I confine myself to groups to which I have paid 

 some attention, leaving entirely out of consideration a large 

 proportion of recent genera. 



(1) The Membraniporidge and Flustrida) are indicated as the 

 most primitive group of Cheilostomes by the relations of their 

 frontal membrane and parietal muscles, and by the evidence 

 afforded by the primary zooecium of many other Cheilostomes. 

 For this group 1 think that Smitt's name Flustrina should be 

 retained. It remains to be seen whether many of the 

 Amphiblestrum-like forms of Membrauipora, in which 

 a calcareous cryptocyst (?) is developed, are more nearly 

 related to this group or to the Microporoid series. 



