HALLIRHOA. 69 



Hallibhoa AQATiiciFORMis, Benett, sp. (Plate XV. figs. 1, 1 a, 1 b.) 



1831. Polypothecia agariciformis, Benett, Cat. Org. Rem. Wilts, t. 15. f. 1, 2. 



^ 1847. Siphonia acaulis, Michelin, Icon. Zoophy. p. 139, t. 38. f. 2. 



1847. Nou Hallirhoa brevicostata, Mich. Icon. Zoophy. p. 127, t. 31. f. 2. 



1854. Hippalimus fungoides, Moms, Cat. Brit. Foss. p. 28. 



Sponges more or less depressed, conical, or mushroom-shaped, with generally an 

 even slope from the cloaca to the exterior margin, which is frequently irregular in 

 outline, more or less circular, occasionally thin, and not infrequently with finger- or 

 spur-like processes extending outwards. The under surface of the sponge is hollowed 

 out and intersected with very prominent sinuous ridges, distributed in an irregular 

 manner. In many specimens there are no traces of u stem, and the sponge is 

 apparently sessile ; in others, however, there is a small cylindrical process in the 

 centre of the underside, which may represent the upper portion of a slender stem. 

 There is considerable variation in the size of different examples — a small specimen 

 measures 40 mm. in height by 75 mm. in width, and a large form is 80 mm. in height 

 and 155 mm. across the base. 



The cloaca is cylindrical and, as a rule, does not extend deep into the body. It 

 is situated at the summit of the cone, its margins are sharp, and its interior surface 

 is covered with the closely set apertures of canals, which, in some examples at least, 

 are disposed in vertical rows. The upper surface of the sponge is covered with 

 numerous, slightly sinuous, branching canals, I'o mm. in width, which radiate down- 

 wards from the margins of the cloaca towards the exterior edge. These canals are 

 generally open, but were in all probability covered by a dermal layer. In a vertical 

 section canals similar in size and direction to those of the surface are exposed. The 

 basal surface of the sponge exhibits numerous irregularly shaped canal-apertures ; 

 there are also indications of a dermal layer extending over tlie under surface. 



The spicules of the interior mesh have robust smooth arms, with tuberculatcd 

 extremities which interlock together. The arms of the spicules are '38 mm. in 

 length and -078 mm. in tliickness. A small portion of the surface of a specimen 

 belonging to the Jermyn-Street Museum is covered with a dermal layer of minute, 

 flattened, deeply laciniated spicules about -65 mm. in width, which are so arranged 

 that their arms overlap each other. These spicules are furnished with shafts which 

 extend into the sponge. I have not been able to detach any of the spicules to 

 ascertain the length of the sliaft, but its presence is clearly indicated by the minute 

 aperture which is seen where the heads of the dermal spicules have fallen oft' the 

 surface. Where this dermal layer is present tlie surface of the sponge is smooth, 

 and the underlying canals are quite concealed from view. 



The examples of this species appear to have been abundant in the Upper Green 

 Sand of certain localities. Miss Benett's figures very accurately represent the outer 

 surface of this species ; notwithstanding this, Michelin relegated her forms to his 



