Folyzoa of the Adriatic. 313 



Supplement. 



Family Membraniporidse. 



Group a. 



Flustra, Linnreus. 



Flustra tenella,n. sp. (PI. IX. fig. 1.) 



Zocecia in two layers, elongate, rectangular, narrow, mar- 

 gins thin and smooth, on each side a little below the top a 

 single short acuminate spine. Avicularia distributed amongst 

 the cells, placed obliquely on a small oblong area, ovate, 

 broad and rounded below, tapering slightly upwards, elevated 

 towards the upper (or mandibular) extremity, and depressed at 

 the opposite end; mandible broadly triangular, bluntly pointed 

 above. Ocecium small and shallow, depressed and covered 

 by the membranous cell-wall, oral arch rather prominent ; 

 the opening closed in by a membranous curtain. Zoarium 

 composed of very narrow segments, expanding slightly up- 

 wards, and bifurcating frequently ; the terminal segments 

 rounded at the extremity. 



This is the form which in the earlier portion of this paper 

 1 referred to F. securifrons^ Pallas ; but further examination 

 has shown me that it is in many important particulars very 

 distinct from that species, and must be separated from it. 

 F. tenella is of very delicate habit as compared with Pallas's 

 species, the segments of its zoarium being very much nar- 

 rower than those of the latter (only about a third of the 

 width), and wanting altogether the broad wedge-like termina- 

 tions that are so characteristic of F. securifrons. In shape 

 the cells of the two forms are very similar, but those of 

 F. tenella are furnished with a pair of spines near the upper 

 extremity, which are altogether wanting in the other species 

 (PI, IX. tig. 1). The most striking differences, however, are 

 found in the ooecium and avicularium. The ooecium in F. 

 securifrons is ample, projecting a considerable way into the 

 cell above it, whilst in the present species it is insignificant in 

 size and very shallow ; in the former the opening is protected 

 by two curious rib-like appendages (PL IX. tig. 2), which 

 originate one on each side and meet in the centre ; in the 

 latter these are wanting and the orifice is simply closed by 

 the usual membranous curtain. The avicularium of /*^. tenella 

 is large, always placed obliquely, with a bluntly [)oiuted 

 triangular mandible of considerable width ; that of F. securi- 

 frons is smaller, with a comparatively short rounded mandible, 



Ann. & Mag. N. Hist. Ser. 5. Vol. x\x. 21 



