130 Transactions of the Sodettf. 



and the whole skeleton, losing all traces of its original composition, 

 exhibits simply a reticulation of solid fibres radiating from equally- 

 solid simple knots. The young fibres are at first smooth, but very- 

 early, almost as soon as they become optically simple, they become 

 tubercled, and with age the tubercles increase in number and size. 



Secondarij rete. — Aitei the formation of the adult network, 

 changes appear to take place in the distribution of the canals of the 

 water-system, by which some of the large meshes become no longer 

 needed as water-channels, and so are gradually filled up by a second- 

 ary network, of what might appropriately be called " darning " 

 fibres, from the way in which they seem to mend up the gaps in 

 the aged skeleton. In one case I found this secondary network in a 

 very early stage (Plate YIII., Fig. 7), its component spicules 

 having only just become soldered together by silica, and difiering 

 considerably in appearance from the budhke spicules, or pullulating 

 fibres of Bowerbank, which likewise unite into a secondary network. 

 As the secondary fibres thicken with the continual deposit of silica 

 over them, they produce a network of a very difierent appearance 

 to that of the principal skeleton, its fibres are more rodlike, often 

 sharply and conically spined, less thickened at the nodes, and 

 sometimes more rectangularly arranged. Contrast, for example, 

 Fig. 7, which is somewhat like the network of a Cypellia (Zittel), 

 or the spined fibre of Fig. 6 with the excellent figure of the ordinary 

 skeletal reticulation given in Bowerbank's Memoir, plate i., loc. cit. 



Other sinctdes besides sexradiates which become involved in the 

 siliceous fibre. — That the large acerate spicules may sometimes con- 

 tribute to the skeletal network has already been mentioned, but I 

 have never before met with an instance in which a flesh-spicule 

 became eo involved. Such a case, however, is shown in Fig. 1 9, 

 Plate VII., where two flesh-spicules are seen closely attached to the 

 surface of a skeletal fibre : in one the process of envelopment has 

 not gone so far as in the other, so that, although the angles between 

 its rays have been to a great extent filled up, yet its characteristic 

 form is more nearly retained, and the rays attached to the fibre are 

 still so far unenveloped as to allow the light to shine through 

 between them ; the other, on the contrary, has become converted 

 into a mere globular tubercle, with the yet uncovered ends of its 

 rays projecting as little spines. 



In commenting on the foregoing descriptions, one may first 

 point out the analogy which exists between the rude folding of the 

 walls in Dactylocalyx and the more perfect folding in such extinct 

 forms as Ccehptychium and the Ventriculites. The resemblance 

 between Coeloptychium and Dactylocalyx appears to be especially 

 marked ; in both we have radial ridges and gullies, of about the 

 same size in each ; in both the ridges are bifurcated, anastomosed 

 laterally, and marked on the exterior with rounded openings leading 



