Fossil Sponges, 128 



(J) Leucones. — The spicules in this family are loosely and Sponges. 

 irregularly distributed in the soft tissues. Sponges of this family GALLERY 

 are very abundant in recent seas, but extremely rare as fossil, and ■vjreBt Side 

 the only ones yet discovered in the rocks are some diminutive Wall-cagee 

 specimens of Leucandra Walfordi, from the Middle Lias, which 7 & 8, 

 are not more than 2 to 4 mm., or about i inch, in height, andgaggg" 

 about 1 mm. or -2V inch in thickness. 11-16. 



{c) Sycones. — In this division the spicules are so arranged as to 

 form radial tubes which o-pen iato the cloa,cal cavity. Fossil forms 

 are rare, and those assigned here are not definitely similar to the 

 living sponges of the group. They include Protosycon, Zitt., and 

 Barrouia, Mun. Chal. 



GEOLOGICAL DISTRIBUTION OF FOSSIL SPONGES. 



As the fossil sponges in the Museum have been arranged in 

 stratigraphical order, their distribution in the different horizons 

 of the geological series can be readily seen by an examination of '^*^^®" 

 the contents of the cases in regular sequence. 



Paleozoic. — Beginning at the base of the series, we find the 

 oldest known sponge, Protospongia feneatrata, Salt., from the 

 Cambrian rocks of St. Davids, South Wales. It belongs to 

 the Lyssakine Hexactinellids ; the only portions known are small 

 fragments of the siliceous meshwork now replaced by pyrites and 

 inclosed in black shale. 



From the Ordovician there are the anchoring tufts of Hexac- .^^jj_ 

 tinellid sponges and the remarkable Hexactinellid Brachiospongia case 8. 

 digitata from Kentucky. 



The Silurian sponges of this country are represented by the 

 Hexactinellid genus Plectoderma, and the aberrant and somewhat 

 doubtful Amphispongia ohlonga. From the same horizon in North 

 America, are found the Monactinellid Cltmacospongia, and the 

 numerous examples of the Lithistid genera Astylospongia, 

 Palceomanon, and Hindia. The Octactinellid genus Astrceospongia 

 comes from Tennessee and the Isle of Gothland, and from this 

 latter place the Lithistid Aulocopium. 



From the Silurian and Devonian strata of Belgium, North TaWe-^ 

 America, and this country, are many examples of Receptaculttes, 

 Ischadites, and Sph^rospangia. Ciists of the Hexactinellid genus 

 Dictyophyton are from the Devonian of North America. 



