294 SIDNEY F. HARMER. 



calcareous roof, wliicli extends to about tlie middle of the 

 orifice, where it ends in a prominent shoulder on each side, 

 rising in the middle line into a massive suboral umbo. This 

 bears a strong avicularium at its base, immediately over the 

 entrance to the compensation-sac. The rounded mandible, 

 when closed, is directed towards the end of the umbo, and 

 then lies in a vertical plane, transverse to the long axis of 

 the zooecium. The umbo, which probably belongs to the 

 avicularium, is supported by a series of radiating buttresses 

 of the fiontal shield, and between them are deep pits, the 

 marginal areoke (ar.), closed by a layer of living tissue or 

 epitheca. 



Fig. 11 shows the commencement of the calcification of the 

 frontal shield. The lateral partition-walls consist of a thin 

 chitiuous lamelhi, with a layer of calcareous matter on each 

 side, belonging respectively to the two zooscia separated by 

 the wall. At a level a little lower than the free edge, the 

 lateral and proximal partition-walls give off a calcareous film, 

 which in the young specimen figured reaches no further than 

 the edge of the frontal membrane. This film is pierced by a 

 series of round holes (jJ.), one of which corresponds to each 

 interval or areola {ar.) between two of the futuie buttresses. 

 These holes establish a continuity between the living tissue 

 of the botly-cavity and that which occurs on the outside of 

 the frontal shield. Beyond the origin of the frontal shield 

 the vertical wall splits into two uiembranous lamellie, one of 

 which passes over each of the contiguous zooccia to form its 

 epitheca, and becomes continuous with the free edge of the 

 frontal shield (fig. 12). The frontal membrane (/. in.) becomes 

 covered by a continuation of this process — in other words, by 

 the formation of a cresceutic fold, of which the deeper 

 lamella is calcareous, and the su])erficial layer is composed of 

 a liviug membrane. In the incompletely calcified zuix-cium 

 the edge of the crescentic fold is alwtiys membranous. I'rom 

 the deep lamella rise the radiating ridges which foini the 

 buttresses. 



The polypide is still young in fig. 11. With the distiil end • 



