SPONGES. 97 



Paratetilla, n. gen. 



Tetillidse with a special layer of modified trisenes, resembling calthrops, lying at the 



junction between the ectosome and choanosome (or in the ectosome). 

 The discovery by Professor HEKDMAN of another species of Tetillid which shares 

 the remarkable skeletal peculiarities of CARTER'S Tethya merguiensis (21), appears to 

 justify the establishment of a new genus for the reception of the two. Paratetilla 

 merguiensis is, like our species, an inhabitant of the Indian Ocean (Mergui Archi- 

 pelago), but its area of distribution extends, according to SOLLAS (15), as far as Torres 

 Straits. The synonymy of this species is, however, somewhat involved, according to 

 LINDGREN (86) and THIELE (87) it should be known as Tetilla bacca (SELENKA), with 

 which, according to THIELE, KIESCHNICK'S Tetilla ternatensis, T. amboinensis, 

 T. violacea, arid T. rubra are again synonymous. If these identifications be correct 

 the range of the species must be yet further extended. 



Paratetilla cineriformis,* n. sp. Plate III., fig. 7. 



This species is represented in the collection by four specimens, ranging in diameter 

 from about 12 millims. to about 41 millims. The shape is irregularly hemispherical. 

 All but the smallest have been injured below by tearing off from the attachment, the 

 base of the sponge having evidently been left behind. The smallest, which appears 

 to be fairly perfect, is cushion-shaped, flat beneath and convex above. The surface is 

 more or less uneven, and to some extent hispid, though not very markedly so ; it 

 may be very irregular. The consistence is rather soft and spongy. The colour 

 externally (in spirit) is almost black ; internally much paler, purplish grey. The 

 arrangement of the vents and pores is very difficult to determine. A few small, 

 pocket-like depressions, irregularly scattered over the surface, probably represent 

 cloacal cavities ; their floors are perforated by the numerous minute apertures of 

 slender, exhalant (?) canals ; their external openings may be narrow and slit-like. 

 The inhalant pores are probably scattered irregularly over the surface, but I have not 

 been able to make them out distinctly. Some of the specimens are infested by 

 parasitic cirripedes, living beneath the surface in cavities whose external apertures 

 simulate vents. 



The main skeleton consists of stout bands of spicules radiating from a large central 

 " nucleus." In addition to these there is the layer of modified trisenes lying at the 

 junction of ectosome and choanosome. 



Spicules. (1.) Modified triaenes (Plate III., fig. 7, a-d), lying at the junction of 

 ectosome and choanosome. These spicules are so irregular in size and shape that it 

 is almost impossible to describe them ; the following have been measured : (a.) with 

 four simple unbranched rays, one much shorter than the others, resembling a plagio- 

 triaene with shaft shorter than cladi ; length of longest ray 0'165 millim., with two 



* The specific name is derived from the close resemblance which the sponge bears to a black cinder. 



O 



