86 Rev. T. Hincks's Critical Notes 



the ribbed condition of the front wall (as in Membraniporella 

 and Crihrilina), the chambered condition of the cavity (as in 

 Steganoporella), &c. One of the most constant features of the 

 zccecium, too, is to be found (as noticed long ago by Hassall) 

 in the orifice, which exhibits a series of well-marked modifi- 

 cations, and has in some cases a developmental history, which 

 affords the most valuable, because the most significant, cha- 

 racters " *. 



In point of fact a very considerable proportion of the 

 Cheilostomatous genera which I have constituted or adopted 

 are not based on " the form of the orifice." The following 

 may be instanced in addition to those referred to in the above 

 passage : — S^honojwrella, a Membraniporidan form charac- 

 terized by a calcareous tubular structure attached to the 

 lamina immediately below the aperture ; Euthyris, also Mem- 

 braniporidan, based on the structure of the operculum, which 

 marks a distinct advance upon the typical Membranipora ; 

 Microptora, SmittijJora (Jullien), Thalamoporella, Setosella; 

 Microporella, founded on the form of the orifice in combination 

 with the "special pore;" Porina, Anarthropora, Mastigo- 

 phora ; Aspidostoma and Rhyncltopora, both based on the 

 remarkable structures connected with the orifice, not on the 

 mere form of it ; Stolonella, allied to Beania, but having the 

 membranous front-wall of the boat-shaped zocecium protected 

 by modified spines, which are united so as to form a con- 

 tinuous covering. Others might be added, but these are 

 amply sufficient to show that, although in certain leading 

 groups the stress has undoubtedly been laid on the form of 

 the orifice, as being at once the most stable and significant 

 character at present, available, there has been no intention of 

 basing the classification generally (as in the old zoarial 

 systems) on a single character. 



As to the zocecial orifice, I believe that it has an intrinsic 

 systematic value, and will probably always hold a distinct 

 place as one of the criteria of affinity. In those sections at 

 least of the Cheilostomata in which the oral opening has lost 

 the primitive simplicity of the Membraniporidan type and is 

 closed in by a solid frame, in which a well-organized oper- 

 culum works on a distinct hinge, this structure has an 

 undoubted significance of a very high order. Smitt, after an 

 elaborate study of the modifications of the zooecial orifice and 

 •the relation between the principal forms of it, felt himself 

 justified in assigning it the foremost place as a generic cha- 

 racter. Even Jullien, who makes the " front-wall " the 

 corner-stone of his system, admits the significance of the oral 

 * Hist. Brit. Mar. Pol., Introduction, pp. cxxix, cxxx. 



