CALCAREA PHARETRONIDAE 



193 



" Tuning-forks," though not exclusively confined to Pharetronids, 

 are yet very characteristic of them. 



Fig. 86.^Portions of the skeleton of Petrostroma schuhei. A, Framework "vvitli en- 

 sheathing 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. 



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

 recently described fossil forms. Petrostroma schulzei ^ lives in 

 shallow water near Japan ; Plectro- 

 ninia halli ^ and Bactronella were 

 found in Eocene beds of Victoria ; 

 Porosphaera^ long known from the 

 Chalk of England and of the Con- 

 tinent, has recently been shown by 

 Hinde^ to be nearly allied to Plectro- 

 ninia ; finally, Plectinia * is a genus 

 erected by Pocta for a sponge from 

 Cenomanian beds of Bohemia. 

 Boeder lein, 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 (Eig. 87) and Porosphaera by 



1 Doederlein, Zool. Jahrb. Abth. Anat. x. 1896, p. 15, pi. ii. and iii. 



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

 » Hinde, Tr. B. Micr. Soc. 1904, p. 3. 



VOL. I 



Fio. 87. — A spicule from the skeleton 

 framework of Plectroninia, show- 

 ing the terminally expanded rays. 

 (After Hinde.) 



* Poeta, Bull. Acad. Bohime, 1903. 

 



