PLATE XXIV. 



Fig. 1. Toichopoma obliquum. 



Fig. 1. A young hydrotheca. The thin roof and the curve which 

 bounds the later operculum are seen, x 50. 



Figs. 2 — 6. Cuspidella procumbens. 



— 2. A piece of a stolon with two typical hydrothecae. x 50. 



— 3. A young hydrotheca. x 50. 



— 4. A somewhat more developed hydrotheca. The lines along 



Avhich the operculum will fold later are shown, x 50. 



— 5. A full-grown hydrotheca ; the operculum opened, x 80. 



— 6. A full-grown hydrotheca; the operculum is bounded from 



the remaining part of the hydrotheca by a sharp edge, x 80. 



Figs. 7 — 14. Calycella syringa. 



— 7. A young hydrotheca ; the hydranth-bud nearly fills out the 



calycle. X 50. 



— 8. A young hydrotheca; the hydranth-bud is withdrawing from 



the retracted roof, in which the radiating lines are seen, x 50. 



— 9. do. The roof artificially turned out. x 50. 



— 10. A young hydrotheca with a low roof; from Iceland, x 50. 

 — 11. A hydrotheca from N. E. Greenland, showing the tongues 



rounded by wear, x 50. 

 — 12. A hydrotheca from Hellebæk, showing the angular tongues. x50. 

 — 13. do., renovated thrice ; the youngest operculum closed, the 



preceding with angular tongues, the two oldest with worn 



tongues. X 50. 

 — 14. A closed operculum seen nearly from the top; the two folds 



on the right are seen edgewise, x 80. 



Fig. 15. Tetrapoma quadridentatum. 



— 15. A young hydrotheca seen from the top; the two crossing 

 lines are seen, which bound the four valves in the not yet 

 opened operculum, x 80. 



