356 Dr. G. J. Hinde on a true Leuconid Gahisponge 



establislied by Ha3ckel has previously been known. So fragile 

 and apparently unfitted to be preserved as fossils are the 

 structures of recent Calcisponges, that HfBckel did not think 

 entire forms would ever be found in the rocks, tliough possibly 

 their microscopic detached spicules might be met with 

 (Kalksch. Bd. i. p. 341). And yet, by some most favourable 

 combination of circumstances, this remarkable fossil Calci- 

 sponge, as fragile as any of its existing relatives, has been 

 preserved since Liassic times. Since Hseckel's Monograph 

 appeared in 1872 numerous fossil Calcisponges have been 

 determined by v. Zittel and others ; but all of them, with the 

 exception mentioned above, belong to the extinct family of 

 the Pharetrones, characterized by having a skeleton of solid 

 spicular fibres. This structural type of Calcisponge appears 

 to have been singularly Avell adapted for fossilization, since 

 sponges of this group are recorded from Devonian strata up- 

 wards ; but as regards some of the older forms, from the 

 Devonian to the Triassic, further evidence of the nature of 

 their tibrous skeletons is still required. Tlie skeletal libres in 

 the Jurassic and Cretaceous Pharetroues, however, consist of 

 spicules closely resembling those of existing Leucones and 

 other recent Calcisponges, and on this ground v. Dunikowski 

 placed them as a mere subfamily of the Leucones (' PaltBon- 

 tographica,' Bd. xxix. (1883) p. 34 sep. Abdr.), and believed 

 that the solid fibres were of secondary origin, produced by 

 fossilization. This view is clearly untenable, since the spicules 

 in the solid fibres of the Pharetroues have oftentimes a very 

 definite arrangement, quite impossible to have been produced 

 by mechanical influences from the irregularly intermingled 

 spicules of Leuconid sponges. We now know from this Lias 

 fossil that sponges with true Leuconid structure date as far 

 back in geological time as any Pharetroues with definitely 

 ascertained spicular fibres; and it. is not improbable that both 

 groups may have coexisted from the Pakeozoic era. It is 

 worthy of note that whilst the Leuconid type still flourishes 

 and is world-wide in its distribution, the Pharetronid type 

 seems to have wholly died out, the latest known * occurring 

 in the Upper Chalk. 



Distribution. The fossils were obtained by Mr. E. A. Wal- 

 ford, P.G.S.f, ill a bed belonging to the Marlstone of the 



* An Australiau Calcisponge, Leucetta clatJirata, Carter (Ann. & Mao-. 

 Nat. Hist. ser. "5, vol. xi. (1883) p. 33), was originally described by Mr. 

 H. J. Carter, F.R.S., as possessing solid spicular fibres ; but lie has since 

 discovered tbat the tibres are really tubular (ib. vol. xvii. (1886) p. 508). 



t I wish to state that the keen observation of Mr. Walford has also 

 brought to hght numerous other small Calcisponges in the Inferior Oolite 

 of Dorsetshire, which are now under examination. They are all Phare- 

 troncs, and iuclud'^ many new species. 



