GEODIA MESOTRIAENA. 



97 



specimen of var. pachana they are more numerous and larger, 15-30 mm. wide 

 (Plate 21, fig. 1). Between the depressions on the upper face and the sides of 

 the latter low elevations, 18-27 mm. broad, arranged in curved rows and forming 

 gyrus-shaped ridges arise. Some of the depressions are isolated and shallow, most 

 of them join to form furrows, 6-12 mm. deep, separating the gyri. At the 

 bottom of these furrows apertures, 5-8 mm. in diameter, which lead into irregular 

 tubes, 4-13 mm. wide, traversing the interior of the sponge and occasionally 

 anastomosing, are observed. These tubes must not be confounded with true 

 canals; they are, as will be shown, praeoscular cavities. Masses of sponge-tissue, 

 on an average 30 mm. thick, separate these tubes from each other. In var. 

 megana and var. microana tubes of this kind are also met with. Here, however, 

 they are wider, in var. megana 20 mm. in diameter, and less numerous (Plate 21, 

 fig. 2b). 



In all the specimens portions of the outer surface (Plate 21, figs. 1, 2a), 

 chiefly the sheltered parts in the depressions, are covered with protruding spic- 

 ules which form a fur. This spicule-fur is 5 (var. microana) to 10 (var. 

 megana and var. pachana) mm. high. The walls of the praeoscular tubes are 

 also hirsute, but here the protruding spicules are less numerous and do not 

 extend nearly so far beyond the surface. I do not doubt that the fur is pro- 

 duced by partial ejection of radial spicules on all parts of the surface, and 

 that, wherever it is now wanting, it has been lost by friction, either during life 

 or post mortem. As stated above, the protruding spicules forming this fur are 

 mostly mesotriaenes. 



Where the large protruding spicules have been lost, slight depressions 1.3 

 (var. megana) to 2 mm. (var. microana) apart are observed on the surface. 

 Apart from these the surface appears, to the unaided eye, quite smooth. 



The superficial parts, abutting on the outer surface and the praeoscular 

 tubes, are differentiated to form a cortex free from flagellate chambers. 



This cortex (Plate 21, fig. 2; Plate 22, figs. 1-7, 10; Plate 23, figs. 24, 25; 

 Plate 25, fig. 1) is composed of three layers: an outer layer, containing small 

 dermal amphioxes and various euasters but no sterrasters, and traversed by 

 systems of mostly oblique canals (Plate 23, figs. 24, 25, Plate 25, fig. 1); a 

 middle layer filled with dense masses of sterrasters and traversed by the distal 

 parts of the narrow, radial, chonal canals (Plate 22, figs. 3-5, 7a; Plate 23, figs. 

 24b, 25b; Plate 25, fig. le); and an inner layer, poor in microscleres, traversed 

 by the proximal parts of the chonal canals (Plate 22, figs. 5, to the right, 6, 7b; 

 Plate 23, figs. 24, 25). All three layers are penetrated by the rhabdomes of the 



