and two Esperladse^om the West Indies. 273 



or weather-worn white colour, while its interior still retains 

 the tawny-yellow one which, in the living condition or fresh 

 state, most probably pervaded it throughout. 



Acarnus tnnominatus, Gray. PI. XVII. figs. 4-6. 



Spiculous, flat, spreading, sessile, penetrating and incrust- 

 ing the interstices of bodies over which it grows, but not 

 boring into them. Colour when fresh unknown, now light 

 grey. Structure delicate. Surface even, isodictyal, present- 

 ing an irregularly hexagonal arrangement of the spicules 

 (PI. XVII. fig. 4). Oscules and pores not seen. Internal struc- 

 ture or skeleton polyhedral, subdivided, consisting of straight 

 lines of spicules supported by delicate sarcode rendered more 

 dense at the angles of union by the addition of the bulbous 

 ends of capitate spicules &c., which project into the interstices 

 (fig. 5). Spicules of five kinds, viz. : — (1) the largest, acuate 

 smooth, slightly curved, and fusiform (fig. 6, a) ; (2) large 

 capitate, shaft smooth, straight, provided with a globular 

 inflation at the fixed end, and an inflated head at the free 

 one, armed with four or five large recurved spines (h) ; 

 (3) small capitate, the same, but mucli less in size, and the sliaft 

 sparsely armed also with recurved sjoines (c) ; (4) tricurvate, 

 bow-like, robust, much arched (d) • (5) equianchoratc, three- 

 fluked, minute {c,f). These spicules are respectively about 25-, 

 18-, 7-, 6- and l-1800th of an inch long. The largest forms the 

 meshes or skeleton of the polyhedral structure (figs. 4 & 5, aaa), 

 and the rest are aggregated at the angles of union, whereby 

 these points are rendered more dense and present a knotted 

 appearance (figs. 4, cc, & 5). Size of largest specimen about 

 an inch square, with variable thickness below l-12th of an 

 inch. 



Hab. Marine. 



Loc. West Indies. 



Obs. This little sponge, apparently of an incrusting habit, 

 humbly creeping over the d(^bris of corals and the like (which, 

 cemented together by calcareous material, appear, from the 

 fragment still attached to Ectyon sparsus^ to have formed the 

 kind of rock on which the latter grew), presents, under the 

 microscope, one of the most beautiful sponge-structures that I 

 have ever seen. Each spicule has a most attractive form; and 

 the whole produce a combination and arrangement (fig. 5) 

 which, for exquisite beauty, individually as well as collec- 

 tively, is, so far as my experience goes, unsurpassed, if not 

 unequalled, among the Spongiada3. 



The hexagonal and isodictyal structure of the surface (fig. 4), 



