98 Kev. T. Hincks's Critical Notes 



from the sides of the ribs, and binding them one to the other, 

 form in fact the only distinguishing character of the group. 

 It is hardly necessary to say that, however interesting as a 

 step in the development of the front-wall, this detail has no 

 special significance and certainly no claim to be adopted as 

 the basis of a family group. 



But, as I have pointed out before, the genus Memhrani- 

 porella is founded on a well-marked type-form, M. nitida, 

 Johnston, in which the spines are transformed into ribs and 

 are no longer isolated, but elements of a well-compacted 

 protective covering, which roofs in the front of the cell. It 

 is at this point in the evolutional series that a new family 

 may be legitimately instituted, not to break the natural con- 

 tinuity of development or obscure the natural relationships, 

 but to mark the morphological advance. 



I am compelled therefore to reject Dr. Jullien's proposed 

 change and to maintain the family Cribrilinidce as at present 

 constituted. 



It must, be remembered that the front-wall in this group is 

 by no means homologous with the front-wall as it exists in 

 most of the Cheilostomatous families. Its mode of growth 

 is different, its constituent elements are different. It is not 

 a continuous extension of the cell-wall, but is formed by the 

 adaptive modification of certain spinous processes which 

 originate on the wall below the margin of the cell. Its func- 

 tion, like that of the solid covering characteristic of other 

 families, is protective, but the two are built on different struc- 

 tural plans and bear different relations to the zooecial 

 organism. A clear indication of this important fact should 

 be included in the diagnosis of the family. 



In the course of a careful study of a large number of Cribri- 

 line species which I have lately made one or two interesting 

 points have been determined, which may be briefly noticed : — 



i. Modification of the Spines. — In the early stages of the 

 Cribriline cell the marginal spines, which are to form the 

 front wall, present the same appearance as the corresponding- 

 parts in a Membranipora. 



They are (in Membraniporella nitida) slender and suberect, 

 but ultimately bend in over the aperture, and increase con- 

 siderably in width by the secretion of calcareous matter round 

 the edges. In this way a broad flatfish rib is formed, in the 

 centre of which the original spine is traceable. This trans- 

 formation of the spine is constant throughout the family. 



ii. The Orifice and Operculum. — The orifice amongst the 

 Cribrilinidce is formed by the two uppermost ribs, which are 

 often stouter than the rest and which shut off and enclose the 



