286 SIDNEY F. HAKMEK. 



arrangemeut of the marginal spines foreshadows the condition 

 above indicated. 



(2) The Cribrilinidee are transitional from Memb ranipora 

 to some at least of the Lepralioid genera. 



(3) The arrangement of the muscles connected with the 

 compensation-sac is derivable from the condition found in 

 Memb ranipora. 



(4) The study of the primary zooecium. 



(5) Palaeontological evidence. 



The view that the Cribrilinidee are intermediate between 

 the Flustrina and the Eschaiina is not a new one. Smitt 

 (1868, i, p. 401) states explicitly that the frontal bars of 

 Membrauiporella nitida are homologous with the free 

 marginal spines of Membranipora lineata; and in his 

 next paper (1868, ii, p. 48) he shows that Cribrilina marks 

 a further transition to the Escharines. He leaves Me nib rani- 

 pore 11a in the Flustrina, while placing Cribrilina in the 

 Escharina. Hincks, on the contrary (1880), places the two 

 genera in the family Cribrilinidas. 



But although taking this view of the intermediate position 

 of the Cribrilinida3, Smitt was not in a position to show in 

 detail how the Flustrino zooecium could be modified into an 

 Eschariue zoujcium. The compensation-sac enters into the 

 question as supplying the clue necessary for the solution of 

 the problem. 



The foregoing instances have given some idea of the 

 relatious of the compensation-sac in its fully developed form. 

 I next proceed to the proposition that the compensation-sac 

 of some Lepralioid genera has been derived from a Mem- 

 brauipora-like condition through a stage similar to that of 

 existing species of Cribrilina. 



(a) Flustrina. 



Flustra pisciformis, Busk.^ — Fig. 4 shows the general 

 anatomy of n young zooecium of this species. In the lateral 



» 1852, p. 50. 



