330 SIDNEY F. HAKMRU. 



In many of the Flustriua there is an elongated frontal 

 memhrane which is tj'pically depressed by a series of parietal 

 muscles. A frontal membrane of this description occurs in 

 the MenibraniporidtB (part), Flustridie, and Farcimiuariidae, 

 and in such genera as Bugula and Beania. 



In other Flustriue forms (Dimetopia_, Bicellaria) the 

 zooecium becomes elongated and usually iufundibuliform, 

 Avith a concurrent reduction in tlie size of the frontal mem- 

 brane — a tendency which mny be shown even in the ances- 

 trula. The reduction of the iVontal surface may be correlated 

 with a reduced number of parietal muscles. 



(2) Another series seems to be constituted by Scrupo- 

 cellaria, Menipea^ and Caberea, a group of "Cellularine" 

 forms in which certain species in each genus have the frontal 

 membrane protected by the excessive development of a single 

 lateral spine, the fornix or scutum, which may, hoAvever, be 

 vestigial or absent. In some cases, as in Menipea 

 jeffreysii/ Norman, the scutum is so largely developed 

 that a complete calcareous frontal wall appears to exist. 

 Round the edge of this is a slit which is the interval between 

 the edge of the oval scutum and the calcareous margin of 

 the frontal membrane. The scutum originates from a broad 

 calcareous base just proximal to the operculum, and on its 

 inner side. It has a nearly straight suboral edge, which 

 meets the basal sclerite of the a2:)proximately semicircular 

 closed operculum. On the outer side the scutum locks firmly 

 into a strong tubercle developed from the edge of the 

 zooecium. If the scutum were to fuse completely with the 

 edge of the frontal surface a compensa.tion-sac would be 

 produced which would liave had a different phylogenetic 

 history from that of many other Cheilustomes. I see no 

 reason why cases of this kind should not exist,- and it is 

 possible that evidence to that effect might be forthcoming in 



' Norman, ' Aim. Mag. Nat. Hist.' (G), xii, 1893, p. MG. 



- The figure given by Waters (1888, pi. i, fig. 1) suggests that this might 

 be the case in Catenaria bicornis; and the structure of C. lafontii 

 (tig. 49) is not irreconcilable with this view. 



