CALCAREA PHARETRONTDAE 



193 



" Tuning-forks," though not exclusively conlined to Pharetrouids, 

 are yet very characteristic of them. 



Fig. 86. — Portions of the skeleton of Petrostroma schulzti. A, Frameworlc -witli en- 

 skeathing 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 hy a cortex containing 

 free spicules. 



The sub-family contains only one living genus and a few 

 recently described fossil forms, Petrostroma schulzei ^ lives in 

 shallow water near Japan ; Flectro- 

 ninia halli ^ and Bactronella were 

 found in Eocene beds of Victoria ; 

 Forosphaera,^ long known from the 

 Chalk of England and of the Con- 

 tinent, has recently been shown by 

 Hinde^ to be nearly allied to Flectro- 

 ninia ; finally, Flectinia * is a genus 

 erected by Pocta 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 Fetrostroma by lateral 

 fusion of the rays, in Flectroninia (Fig. 87) and Forosphaera by 



' Doederlein, Zuol. Jahrh. Abth. Anat. x. 189(5, p. 15, pi. ii. and iii. 

 - Hinde, Quart. Journ. Geol. Soc. Ivi. 1900, p. 50. 



'^ Hinde, Tr. R. Micr. Soc. 1904, p. 3. * PoCta, Bull. Acad. Bohtme, 1903. 



VOL. I 



Fio. 87. — A spicule from the skeleton 

 framework of Plectroninia, show- 

 ing the terminally expanded rays. 

 (After Hinde.) 



