40 0. F. JENKIN. 



(i) Small alate quadriradiates forming the edge. Basal ray 140 /a X 8 /a, 

 straight, tapering uniformly to a sharp point. Paired rays bent slightly 

 downwards, about 70 ju, x 10 /a. Apical ray short and conical, 20 ju. long x 

 8 fji thick at base, perpendicular to facial plane. Oral angle nearly 180°. 



Megapogon pollioaris.* 

 (Plates XXXVTI. and XXXVIII. , Figs. 125-130.) 



There are two specimens of this new species in the collection ; the larger one is fixed 

 in osmic acid and is consequently brown ; the smaller one, which is fixed in alcohol, is 

 white ; both are hispid. The larger specimen is in the form of a flattened tube 15 mm. 

 long. The diameter at the widest part near the middle is 3 mm. The smaller 

 specimen is ovoid, 2| mm. x l| mm. There is a large gastral cavity which terminates 

 in the oscule, which in both specimens is bent to one side. The larger specimen has an 

 oscular collar about 1 mm. long. 



The structure of the body -wall is shown in Fig. 126. The space between 

 gastral and dermal layers is filled up with three or four layers of flagellated chambers. 

 Each flagellated chamber is surrounded by a slender skeleton of minute prickly hastate 

 oxea (see Figs. 129 and 127) ; these oxea often lie together in small bunches of 

 three or four, and as they may lie partly on one and partly on another chamber, they 

 build up serpentine lines of minute spicules threading about between the chambers. 

 Some of these minute oxea also lie in the gastral and dermal layers, and also in the 

 columns supporting the dermis. 



Canal System. — There is an incurrent chamber under the dermal layer which is 

 probably continuous round the whole body of the sponge. The dermal layer is 

 supported over this chamber by columns of spicules consisting partly of the basal rays 

 of the chiactines and partly of oxea which project outside the dermis. The incurrent 

 canals lead inwards from the incurrent chambers. The excurrent canals are connected 

 in groups to excurrent chambers which open into the gastral cavity through large 

 ports ; these are more or less surrounded by the apical rays of the chiactines (see 

 Fig. 125). 



The Skeleton. — The dermal skeleton is formed by a dense layer, five or six 

 spicules thick, of triradiates, pierced at intervals by the projecting bunches of oxea. 

 The body skeleton consists of chiactines and a very few scattered triradiates. The 

 basal rays of the chiactines are bunched more or less together at their distal ends and 

 occasionally project through the dermal layer ; with each bunch are grouped a number 

 of the oxea, which project with their thicker bent ends about one-third of their length 

 beyond the dermis. There is no special gastral skeleton ; the gastral cortex (see Fig. 

 130) is supported by the paired rays of the chiactines, and a few minute oxea 

 irregularly scattered. 



* rollicaris = " like a thumb," uamed from the shape of the oxea. 



