_Lelapia australis_, a recent species, should probably be placed here as the sole living representative. Dendy has shown[223] that this remarkable species has a skeleton of the same fibrous character as is found in typical Dialytinae, and that the triradiate spicules in the fibres undergo a modification into the "tuning-fork" type (Fig. 86, C), to enable them to be compacted into smooth fibres. {193}"Tuning-forks," though not exclusively confined to Pharetronids, are yet very characteristic of them. [Illustration: FIG. 86.—Portions of the skeleton of _Petrostroma schulzei._ A, Framework with ensheathing pellicle; B, quadriradiate spicules with laterally fused rays; C, a "tuning-fork." (After Doederlein.)] II. LITHONINAE.—The main skeletal framework is formed of spicules fused together, and is covered by a cortex containing free spicules. [Illustration: FIG. 87.—A spicule from the skeleton framework of _Plectroninia_, showing the terminally expanded rays. (After Hinde.)] The sub-family contains only one living genus and a few recently described fossil forms. _Petrostroma schulzei_[224] lives in shallow water near Japan; _Plectroninia halli_[225] and _Bactronella_ were found in Eocene beds of Victoria; _Porosphaera_[226] long known from the Chalk of England and of the Continent, has recently been shown by Hinde[226] to be nearly allied to _Plectroninia_; finally, _Plectinia_[227] is a genus erected by Počta for a sponge from Cenomanian beds of Bohemia. Doederlein, in 1896, expressed his opinion that fossil representatives of Lithoninae would most surely be discovered. The fused spicules are equiangular quadriradiates; they are united in _Petrostroma_ by lateral fusion of the rays, in _Plectroninia_ (Fig. 87) and _Porosphaera_ by {194}fusion of apposed terminal flat expansions of the rays, and in some, possibly all, genera a continuous deposit of calcium carbonate ensheaths the spicular reticulum. Thus they recall the formation of the skeleton on the one hand of the Lithistida and on the other of the Dictyonine Hexactinellida (see pp. 202, 211). "Tuning-forks" may occur in the dermal membrane. [Illustration: FIG. 88.—_Astrosclera willeyana_, Lister. A, the Sponge, × about 3. _p_, The ostia on its distal surface. B, a portion of the skeleton showing four polyhedra with radiating crystalline fibres. C, an ostium; the surrounding tissue contains young stages of polyhedra. (After Lister.)]