on the Polyzoa. 97 



orifice, through which the polypide issues from its cell, is a 

 simple semicircular opening in the membranous wall, which 

 is closed by a movable valve. 



In another section we meet with an important modification 

 and adaptation of the spinous appendages, resulting in the 

 formation of a true roof-like structure, which give* a new 

 character to the zooecium and marks a great advance upon the 

 slight protection afforded by a number of isolated spines. 

 The spines are now represented by broad flatfish ribs, which 

 bend in over the aperture, those on each side meeting in the 

 centre of the cell, where their free extremities are firmly 

 soldered together. Laterally they remain separate, and the 

 fissures between them are filled in by the primitive membra- 

 nous wall. This group is the genus Membraniporella of 

 authors. In it a well-framed protective covering, in great 

 part calcareous, has been superadded to the simpler structure 

 of the true Membraniporce — a most significant morphological 

 advance. 



If we proceed a step further we find that in other kindred 

 forms the ribbed front-wall is strengthened and consolidated 

 by the addition of small lateral offsets (calcareous) from the 

 ribs, which stretch across at short intervals from one to the 

 other, and so bind them together and strengthen the fabric. 

 The spaces between these intercostal supports are usually occu- 

 pied by a line of pores. The genus Cribrilina has been 

 founded for this well-marked structural type ; and the two 

 last-named genera constitute the family Cribrilinidce, as I 

 have defined it, of which the distinguishing character is that 

 the zooecia possess a ribbed calcareous front-wall, more or less 

 consolidated, a character which has no existence amongst the 

 true Membraniporce. 



Dr. Jullien, as we have seen, affirms that the genus Mem- 

 braniporella includes species which have a calcareous front- 

 wall and others in which the marginal spines are not abso- 

 lutely soldered together on the median line (' Costulide'es,' 

 p. 1). On what characters then, we may ask, is the genus 

 founded, and by what criteria is it distinguishable from Mem- 

 branipora ? By admitting that species which have the spines 

 thoroughly united along the median line may mingle in the 

 same group with others in which they are not " absolutely " 

 united (that is, I presume, not really united at all), he virtu- 

 ally destroys the foundation on which the family Gribrilinidce 

 has hitherto rested. What remain, then, as the distinctive 

 features of his own Costutidce? The ribbed calcareous front- 

 wall is also a character of his Membraniporidan genus Mem- 

 braniporella. The small processes (" trabecules ") given off 

 Ann. & Mag. X. Hist. Ser. 6. Vol. v. 7 



