PRINCIPAL GROUPS OF THE SPONGES. l8l 



composed of minute filiform triradiate spicules. In Elasmostoma 

 (fig. 7 1 , d) the fibres of the skeleton are composed of large three- 

 rayed and four-rayed spicules, arranged in the centre of the fibre 

 and surrounded by smaller sinuous spicules. Lastly, in the genus 

 Pharetrosfiongia, regarded by Professor Sollas as a Halichondroid 

 Sponge, the skeleton-fibre is composed of minute, straight or slightly 

 curved, uniaxial spicules, arranged parallel with one another in the 

 direction of the fibre. 



The latest known representatives of the Pharetrones occur in the 

 Maestricht Chalk, but no examples of this group of sponges have 

 been as yet detected in any strata of Tertiary age, nor are any re- 

 cent forms recognised which could be referred to this remarkable 

 division of Calcisponges. 



LITERATURE. 



i. " Porifera." Bronn's ' Klassen und Ordnungen des Thier-Reichs,' 

 vol. ii. (New Ed.), 1882-87. G. Vosmaer. 



2. " Die Kalk-Schwamme." Haeckel. 1872. 



3. " Handbuch der Palaeontologie," vol. i., part 2, 1879. Zittel. 



4. " Studien iiber fossile Spongien." ' Abhandl. d. k. bayer. Akad. der 



Wiss.' 1877 and 1878. Zittel. (The three important memoirs 

 published by Zittel under the above title are translated into 

 English by Mr W. S. Dallas in the "Annals and Magazine of 

 Natural History," for 1877, 1878, and 1879). 



" Beitrage zur Systematik der fossilen Spongien." ' Neues Jahrbuch 

 fur Mineralogie,' &c, 1877, 1878, 1879. Zittel. 



" Ueber Cceloptychium. Ein Beitrag zur Kenntniss der Organ- 

 isation fossilen Spongien." 'Abhandl. der k. bayer. Akad. 

 der Wiss.' Bd. xii. 1876. Zittel. 



" Ueber Astylospongidae und Anomocladina." ' Neues Jahrb. fur 

 Mineralogie,' &c, 1884. Zittel. 



" Die Spongitarien des Nord - deutschen Kreide - gebirges." ' Pal- 

 aeontographica,' 1864. F. A. Roemer. 



" Paleontologie de la Province d'Oran." 5th Fasc. " Spongiaires," 

 1872. Pomel. 



10. " Lethaea Palaeozoica." Ferdinand Roemer. 1880. 



11. " Ventriculitidae of the Chalk." 'Annals Nat. Hist.,' 1847-48. Toul- 



min Smith. 

 " Beitrage zur Kenntniss der Organisation und systematischen 

 Stellung von Receptaculites." C. W. Giimbel. 'Abhandl. der 

 k. bayer. Akad. der Wiss.' Bd. xii. 1876. 



13. "Catalogue of the Fossil Sponges in the British Museum." 1883. 



Hinde. 



14. "A Monograph of the British Fossil Sponges." Palaeontographical 



Society, 1887-88. Hinde. 

 " On the Structure and Affinities of the Family Receptaculitidae." 

 'Quart. Journ. Geol. Soc' Vol. xL 1884. Hinde. 



16. "On Beds of Sponge-remains in the Lower and Upper Greensands 



of the South of England." ' Phil. Trans.,' 1885. Hinde. 



17. " Notes on Fossil Calcispongiae." 'Ann. & Mag. Nat. Hist.,' 1882. 



Hinde. 



