332 SIDNEY F. HARMER. 



layer of the spines is now represented only by a membranous 

 epitlieca_, composed of living tissue bounded by an external 

 cuticle, Avhile the frontal shield itself corresponds with the 

 deeper layer of the original series of spines (fig. 12). The 

 frontal shield extends to the sides of the operculum as a pair 

 of calcareous shoulders, which commonly bear avicularia, 

 while the suboral part of the shield may support a median 

 avicularium which guards the entrance to the incipient com- 

 pensation-sac. " Mucrouella^' pavonella, Alder (p. 29G), 

 probably belongs to Umbouula, which may also include 

 M. bicuspis, Hincks.^ 



So far as I can judge fi'om dry preparations, the develop- 

 ment of the frontal shield takes place in the Umbonuloid 

 manner in Porella compressa, P. saccata" ( = ]\ 

 elegantula, auctt.), P. (Palmicellaria) skenei, Mucro- 

 nella coccinea, and Escharoides sarsii. In specimens 

 of U. verrucosa from Naples^the suboral avicularium has a 

 mandible which closely resembles that described by Waters* 

 in species of Porella, being of more or less semicircular 

 form, with denticulated margin and conspicuous A-shaped 

 main sclerite. This confirms the association of Porella with 

 Umbonula. I am inclined to think that the presence of 

 marginal areolte (as shown in figs. 10, 11) is of great import- 

 ance as indicating affinities in this direction. 



(6) In the Lepralioid type of calcification (figs. 13 — 15, 41) 

 the young zooecium frequently has a Flustrine appearance ; 

 but the frontal shield seems to result from a direct calcifica- 

 tion of the frontal membrane. The compensation-sac is 

 developed, after calcification is complete, as an invagination 

 formed at the base of the operculum. This is the case in 

 Lepralia, Sch izoporella, Urceolipora nana, Euthy- 

 ris, Euthyroides, Vittaticella, and Catenicella. The 



' 'Ann. Mag. Nat. Hist.' (5), xi, p. 201. 



2 Walers, 1900, p. 81. 



^ It is, however, possible that this form is not identical with the littoral 

 form common on tiie south coast of England, which 1 have alluded to as U. 

 verrucosa. 



< iiiOO, pi. xi. 



