I. Dialytinae.—The spicules are not fused to one another; the exact mode of their union into fibres is unknown, but an organic cement may be present.
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. "Tuning-forks," though not exclusively confined to Pharetronids, are yet very characteristic of them.
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.
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 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.
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.)