98 DlCTYOSPONGID^. 



Tliese nodes lie at regular intervals along the margins of appressed 

 })rismatic longituditial faces, and, presumably, at the intersection of such ridges 

 Avith horizontal spicular bundles of the first order. Instances of irregularity 

 in the development of these nodes are very frequent, indeed it is rarely that the 

 normal number and order of the nodes is retained in full-grown examples. 



Reticulum. The specimens of this species are preserved as external and 

 internal casts in sandstone. The very extensive material which has been 

 imder examination has not furnished a single instance in which the parts of 

 the spicular skeleton have been indicated except as impressions in the sand- 

 stone. Such impressions are, lu)weA'er, very sharp, and afford a complete 

 conception of the gross structure of the skeleton. The sui'face is very I'egu- 

 larly i-eticulated by vertical and hoiizontal series of spicular bands. The 

 vertical bands of the first order are, as already observed, those along which 

 the vertical rows of nodes are arranged and correspond to the angles of the 

 elementary prism-faces ; the horizontal bands passing over the nodes do not, 

 however, appear to be perceptibly larger than their neighbors. The general 

 appearance presented by the surface is that of regular quadration by sub- 

 equilateral rectangles, whose size varies with the individual, and the number of 

 which between any two nodes in a vertical row also appears to be an indi- 

 vidual peculiarity. In a slender example, one of these quadrules measures 

 appi-oximately 5 mm. on each side and there are nine of them between two 

 nodes, while in a very I'obust individual they measure about 10 mm. on each 

 side and there are six between successive nodes. 



When the surface is Avell preserved, four distinct series of rectangularly 

 reticulating bands may be observed within each of these quadrules. External 

 casts of the cups, broken in a favorable manner, show that the vertical bands 

 connecting the nodes were produced beyond the surface, and this we may 

 infer to have been the condition of all the larger bands ; thus producing an 

 exterior somewhat similar to that of Clatukospongia, though these lamellae 

 are broader in the latter genus. As these longitudinal lamellae rise over the 

 bases of the nodes they become much broader, expanding to form a flattened 

 triangular tuft at the apex of each node, often greatly extended, and consider- 

 ably broader than the depth of the node. From the summit of the node, at 

 the center of this triangular expansion, extends a stout bundle of spicules to 

 and beyond the apex of the lamella (see Plate vi, fig. 3). Upon internal casts 

 of H. tuberosum, there is frequently an irregular depi'ession or pit at the 

 summit of each node, and it may be inferred therefrom that the proximal ends 

 of the central tufts have left these cavities (see Plates iv and v). 



