each other; nor could we find specific differences with regard to die canal-system. And yet, 

 two species must be accepted on account of the spiculation, viz. melobesioides and carinata. 

 We wil] first describe the external appearance and the general anatomy of both these species 

 together, and afterwards treat separately on their spiculation. 



I. EXTERXAL APPEARANCE OF PLACOSPONGIA MELOBESIOIDES 

 AXD CARIXATA. (PI. I, fig. 1—9). 



Shape. Fullgrown specimens seem to be always branching. The diameter of the 

 branches is variable ; in some specimens they are of about the sa me size (figs. 3 and 8), in 

 others they differ considerably (hg. ij. Though the branches are usually more or less cylindrical, 

 it happens not unfrequently that they are flattened in one direction (163 a) ! ). In several cases 

 the ramification is markedly dichotomous (1033, 1848, 1849 a, 1849 b), in other cases it is more 

 irregular (311 b, 311 c, 311 e, 311 f); both may be seen in one and the same specimen (1850, 

 1851). Concrescence of neighbourhing branches is often seen. When young, the sponge seems 

 to be incrusting (fig. 4) ; in fig. 3 a crust (a) is seen to have produced lateral branches. Though 

 we have reason to believe that the whole complex of branches in older specimens may grow 

 in an erect position, we are al most certain that in one case the branches have been growing 

 in a horizontal position. This specimen (1847) is illustrated in fig. 8. The bottom on which it 

 was dredged, is said to consist of "sand, small stones and shells." We see our sponge attached 

 to such a mass of sand and shells ; it is of course most probable that such a comparatively 

 loose mass simply lay on the bottom. Consequently the place where the sponge is attached, 

 is the upper part, and the Placospongia, therefore, grew in a horizontal direction. 



Sur face. As no spicula are projecting bevond the surface, this is smooth ; it shows 

 usually very distinct, numerous carinae, which are especially conspicuous if in a longitudinal 

 direction. A closer examination shows that the carinae are the borders of a system of grooves. 

 which form a kind of network on the surface. The meshes of this network are composed of 

 hard cortical plates usually showing a polygonal shape, and somewhat concave. In the ridges or 

 grooves a large number of incurrent apertures (stomata) and a few large, excurrent ones (procts) 

 are situated. The size of the cortical plates is very variable. - - On some parts of the surface 

 small tubercles are visible; these are, at least for the greater part, incipient branches. — The 

 surface is frequently covered with Bryozoa, Tunicata, Algae etc. ; likewise visible on the surface, 

 holes are very often met with, occupied by some species of Cirripedia (fig. 5, a). 



Size. It is rather difficult to judge about the exact size our sponges can reach, for 

 none of the large specimens are quite uninjured. The total height of our largest fragment is 

 36 cm. (1849 a), i. e. almost twice as much as the largest specimen known hitherto. The greatest 

 diameter of the branches does not correspond with this, as will be easily seen from the following 



1 1 These numbers indicate the running numbers of the specimens. 



