104 CEYLON PEARL OYSTER REPORT. 



the contraction of the former of which lias, in almost all cases, completely closed 

 the aperture. The diaphragm contains sigmata, but no other spicules. A similar 

 arrangement of the external apertures of the canal-system appears to be characteristic 

 both of Azorica and Scleritoderma ; indeed, our Ceylon species, except for the absence 

 of microstrongyles, resembles very closely indeed Schmidt's Scleritoderma packardi 

 from the Gulf of Mexico, as described and figured by Sollas in his work on the 

 " Challenger " Tetractinellida. 



K.N. 40 (Stat. XV., Periya Paar, Gulf of Manaar, 9 fathoms). 



Petromica, Topsent. 

 Lithistida of massive form ; with scattered pores and vents ; with thin dermal 

 membrane destitute of special skeleton ; with monocrepid desmas feebly united 

 or quite separate ; with monaxonid rhabdi often collected in fibres which may 

 terminate in surface conuli ; without microscleres. 



This genus was founded by Topsent in 1898 (45) for a new Lithistid sponge from the 

 Azores, very closely related to Azorica. Topsent gives the following diagnosis : 

 " Azoricidse massives, en forme de cones dresses, a surface conuleuse, a pores disperses, 

 a oscules membraneux, a ectosome devellope aspiculeux, a desmas peu ornes et 

 faiblement relies entre eux." 



The discovery of a second species in Ceylon waters, closely related to the type of 

 the genus, forms a further justification for separating Petromica from the other 

 Azoricidee. 



Petromica inassalis, n. sp. Plate IV., fig. 5. 



Sponge massive, may be attached by a broad base, may be compressed vertically 

 or horizontally. Vents usually numerous, rather small, but variable in size ; 

 scattered on the upper part of the sponge, each forming the termination of a 

 vertical oscular tube. Pores scattered. Surface uneven, variable, more or less 

 corrugated ; in places covered with a thin, reticulate, pore-bearing dermal membrane 

 overlying large sub-dermal cavities and supported on bundles of large monaxonid 

 spicules which sometimes terminate in conuli. Texture incompressible, but friable ; 

 colour (in spirit) yellowish grey throughout. The type specimen (Pt.N. 257) measures 

 about 37 millims. in height, the same in breadth, and 24 millims. in thickness. 



The main skeleton is a reticulation (sometimes close and sometimes so loose that 

 the desmas do not touch one another) of much -branched monocrepid desmas, inter- 

 mingled with numerous large monaxonid spicules which are partly collected together 

 in coarse fibres (without being united together by any cementing substance). 

 These fibres, as already noticed, sometimes terminate in surface conuli. There is 

 no special dermal skeleton. 



Spicules. (1.) Monocrepid desmas (Plate IV., fig. 5, a) ; much and very irregularly 

 branched ; not tuberculate (or very slightly so) ; branches usually terminating in 

 short, blunt, conical points, or in curved flattened expansions pressed against other 



