RHABDOPLEURA COMPACTA. 581 



HlIABDOPLEURA COMPACTA. 

 Plate LXXII. Ggs. 8, 8a, 9. 



Zoarium composed of delicate chitinous tubes laid 

 closely side by side, and forming small, crust-like 

 colonies; erect, annulated cells, of very moderate height, 

 rising at intervals from the adherent portions; many 

 dark-coloured somewhat oval bodies, with a chitinous 

 covering, in the course of the axial rod, developed on its 

 upper surface. 



R. COMPACTA differs remarkably from the preceding, and also 

 from the Scandinavian R. mirabilis, in its habit of growth. 

 Instead of a creeping, more or less ramified zoarium, it 

 forms minute, compact crusts, composed of tubes placed 

 side by side, and closely appressed one to the other. 

 The erect cells seem to be short as compared with those 

 of R. Normani ; they are given off, I believe, at intervals, 

 in the course of the adherent tube, as in R. mirabilis, and 

 not merely at the extremity of the branches, and pro- 

 bably rise one from each compartment of the stem. The 

 precise mode in which the adherent tubes are disposed 

 and the cells originate I am unable to determine, as my 

 specimens, which are mere specks on the surface of old 

 shells dredged years ago, are much shrivelled up, and do 

 not show many of the structural details. They leave no 

 doubt, however, as to their true nature ; and the habit of 

 growth is so marked as to separate them from both the 

 known species. The axial cord, which is of a very dark 

 colour, is admirably preserved, as are also certain oval 

 bodies in its course, which are probably the so-called 

 statoblasts of Allman (Plate LXXII. fig. 9). They occur 

 usually just below the expansions of the rod, to which 

 the funiculus was attached. 



