346 SIDNEY F. HAEMER. 



Fig. 19. — Ojjeiculuni and hiljiuni (/.) from a dry specimeii. The arrow 

 indicates the entrance to the compensation-sac. 



Fig. 20. — Part of the frontal surface of a brancli ; ?//., lateral margin of 

 the frond, the epilheca of which covers a continuous marginal cavity 

 (w. c), strengthened by calcareous bars {c. b.) in its frontal wall. Two of 

 the opercula {B, B') are different from the others (A). 



Fig. 21. — The zoojcium B and parts of its neighbours, of the preceding 

 figure ; d. w., distal wall of B ; p. w., proximal wall of its distal neighbour. 

 (For X, see Fig. 30.) 



Fig. 22. — Basal view of several zooecia ; m, mesenttry-like lamella 

 of chitin connecting the proximal part of the basal wall of the zoteeiiim 

 with the basal epitheca. 



Fig. 23. — Labium, seen fium the distal side. 



Fig. 21'. — Open operculum (o/;.) and labium (/Z*.), in side view; com!., 

 chil.inous lamella covering the condyle (decalcified) ; *6'/., sclerite surround- 

 ing the cavity in which the ojiercnium lies; c.s., beginning of floor of 

 compensation-sac ; p^s/., entrance to vestibule. 



Fig. 25. — Closed operculum and labium ; ocr/., occlusor muscles; dio., 

 divaricator muscles. (I'or .?•, see Fig. 30.) 



Fig. 26. — Longitudinal section (thick) of a zoocinm; b. ep., basal 

 epitheca; ep. c, cavity beneath epitheca ; y! (J/., a bar belonging to the 

 frontal shield. 



Fig. 27- — Distal part of the operculum, with the labium {lb.), seen in a 

 thick transverse section of tiie zooecium (not decalcilled). The condyles 

 {cQiid.) are in tiie foreground, and the part of the operculum immediately 

 connected with them (cf. Fig. 29) is not indicated. The labium anil tlie 

 biting edge of the operculum are seen at a much dee|]er locus; p., pores. 



Figs. 28 — 31. — Transverse sections of opercula. 



Fig. 28. — On the distal side of the condyle, showing the great extent 

 of the vertical lateral llanges,' and the free terminations of the buttresses 

 of the operculum. 



Fig. 29. — Through the region of the condyles; the opeiculum is jjar- 

 tially open. 



Fig. 30. — On the proximal side of the condyles ; showing the way in 

 whicii the circular marks (.r) which appear in Fig. 21 are formed. 



Fig. 31. — Immediately distill to the bas.il sclerite. 



1 Owing i() the hardness of the chitin of the opercula, 1 have not succeeded 

 in obtaining completely satisfactory thin sections showing the relations of the 

 labium to adjoining |)arts. 



