110 RECORDS OF THE AUSTRALIAN MUSEUM. 



This species displays considerable variation, and exhibits three 

 marked forms with numerous intermediate stages between them, 

 none of which approach towards Thorecta tenuis or T. erecta. The 

 texture of the latter is not unlike that of T. inaryinalis, but all 

 its remaining characters are different. 



(a) Well developed specimens attain a height of between 200 

 and 300 mm. ; one-fourth of the length consists of a compressed 

 peduncle, twice or thrice as broad as thick ; the expanded base 

 usually has a few root-like processes. The upper three-fourths is 

 generally tongue-shaped, widest above the middle, with the broad 

 anterior and posterior surfaces concave, and the sides either flat 

 or slightly convex. The angles bear seriate and somewhat elongate 

 oscula, at fairly even distances apart ; their shorter diameter is 

 about 2 5 mm., and the longer about 4 mm. The upper extremity 

 is thin, acute, and may either be truncated or pointed. 



{h) This form rarely attains to more than half or two-thirds the 

 height of (a). The peduncle is sharply defined, and is often sub- 

 cylindrical, but usually a little broader than thick. The upper 

 two-thirds of the sponge consists of three or four vertical lamellae; 

 four is the usual number, and a median transverse section would 

 be more or less cruciform. The oscula are seriate and confined 

 to the margins. 



(c) In this form the peduncle is similar to that of the preced- 

 ing. The lamella is strongly and longitudinally concave ; a trans- 

 verse section through 'the middle would be semi-circular, with 

 even inner and sinuous outer margins. The inner surface is 

 smooth ; the outer often bears two or more low longitudinal 

 ridges, and sometimes it is evenly convex. The oscula are con- 

 fined to the convex surface, and are irregularly scattered. The 

 position of the oscula are exactly the reverse of what obtains in 

 Thorecta tenuis and T. erecta, in which they are confined to the 

 concave surface. Another character which tends to show the 

 distinctness of T. erecta is the frequent presence of large oscula; 

 these are wanting in T. rnarginalis, and also in T. tenuis. 



The main fibres contain very large sand grains, and near the 

 surface a few spicule fragments. The sand grains are often 

 isolated, and frequently about O-f) mm. in diameter, whilst the 

 fibre on each side of the grain is only about 01 T) mm. in width. 

 Another peculiarity which is noticed in the original description 

 is the tendency of the secondary fibres to form " trellis-like closer 

 recticulations in the vicinity of the main fibres." The latter 

 feature is not evident in T. erecta or T. tenuis. 



