414 DR S. F. HARMER AND DR W. G. RIDEWOOD ON THE 



of the species grouped in the former subgenus appeared to be of systematic importance. 

 The study of the present material, however, goes some way to break down the distinction. 

 In Cephalodiscus agglutinans the internal system of spaces in the coenoecium is con- 

 tinuous throughout (except for occasional septa, see below), and zooids can move fairly 

 freely through the system, as may be seen by the manner in which they have retreated 

 to the internal parts of the branches in the material under consideration. So far as one 

 can judge from a study of pieces of colony preserved in alcohol, the growth of a branch 

 in a species of Idiothecia is by the buds (or possibly larva?) settling at the apex of the 

 branch, and each secreting a tube of its own, independent of those already existing, 

 whereas in the present species the (presumably young) zooids at the apex secrete tubes 

 continuous with the existing system of spaces within the branch. But C. agglutinans 

 differs from all known species of Demiothecia in having long tubes, i.e. spaces of 

 approximately even calibre extending for considerable distances ; for in the recorded 

 species of Demiothecia there is a single irregular branching cavity in the interior of the 

 piece of colony, and this opens abruptly to the exterior here and there on the surface of 

 the branch, or by means of very short tubular passages (see C. hodgxotii, 07 1 , pi. iv. 

 fig. 22). 



The disposition of the tubes within a branch of the colony may be studied by dis- 

 secting away the soft part of the coenoecium and the included shelly particles, but the 

 process is laborious and not altogether satisfactory in other respects. Much better 

 results can be obtained by decalcifying a branch in a 1 per cent, solution of nitric 

 acid for ten days or a fortnight, and then bisecting the branch lengthwise and dissect- 

 ing out such of the tubes as are laid bare. The process of decalcification removes all 

 the shelly particles, and leaves only the grains of slate (see p. 418), which are not 

 numerous and can be easily picked out. The piece of colony after decalcification is of 

 a dirty brown colour, and is translucent, without the opacity and whiteness which is 

 such a marked feature of the ccencecium before it is placed in acid. In text-fig. 2, A, 

 is shown a diagrammatic representation of the cut surface of such a bisected branch. 

 (Compare this with the diagrammatic longitudinal section of a branch of U. nigrescens, 

 in 07 1 , pi. iv. fig. 10.) 



This diagram (text-fig. 2, A) explains the main features of the tubular passages of 

 the ccenoecium. It shows that in the most superficial parts the tubes are fairly straight, 

 and of uniform diameter, and that they slope more or less radially outwards and incline 

 somewhat towards the apex. The tubes in the middle are irregular in their arrange- 

 ment, but on the whole are set at a uniform distance from one another. What appear 

 to be blind ends of tubes at d and d' are merely parts of tubes that are cut across 

 obliquely because they are leaving the plane of section. 



In some of the passages, especially in the more basal parts of a branch, as at s and 

 5', there are thin concavo-convex partitions or septa, sometimes two or three in close 

 succession. These septa, however, are not common. Their concave faces may be 

 directed towards the apex of the branch, as at s', or away from it, as at s. 



(ROT. soc. EDIN. TRANS., VOL. XLIX., 538.) 



