478 ASTROSCLEBA WILLEYANA, THE TYPE OF A NEW FAMILY OF SPONGES. 



the skeletal elements, in the former the diverging lines are frequently grouped in 

 frond-like systems, whose axes radiate from the common centre. 



d. Even in the same specimen the spherulitic structure may be found in one 

 part of a specimen and the spicular structure in another. Thus Zittel speaking 

 of the spherulitic structure, says : — " Auch dieser Erhaltungszustand kommt zuweilen 

 an Exemplaren vor, bei denen sich einzelne Fasern noch deutlich als aus Nadelu 

 bestehend erweisen." (Studien ilher fossile Spongien, lir. p. 14.) 



e. In a specimen of CoryneUa tetragona tigured by Zittel (PI. XII. Fig. 2) the 

 trabeculae are composed of apparently simple spicules embedded in a matrix, but the 

 spherulitic structure is also present, printed as it were over the other, and with its 

 radiating fibres pursuing their course regardless of the presence of the spicules. 



/. In some of my St Cassian specimens the spherulitic structure Is limited to 

 the trabeculae, but in many cases it extends out from them into the calcite mosaic 

 which fills in the original spaces of the sponge, and in some cases spherulites occur 

 distinct altogether from the trabeculae and isolated in the calcite filling the spaces. 



There is one argument which has been brought forward in favour of the primary 

 nature of the systems making up the spherulitic structure, which deserves further 

 considei'ation. Steinmann describes in the wall of Thauinastocoelia which has this 

 .structure, minute ramifying passages which he believes to be Thallophyte borings, and 

 appeals to their presence in support of the view that the parts which they penetrate 

 had their present structure at the time that the borings were made\ If this con- 

 conclusion were justified, I do not see how the argument could be met. Rauff- 

 doubts the interpretation which Steinmann gives of these markings, and expresses his 

 belief that they indicate the remains of spicules. What their real nature may be in 

 this particular case I am, of course, unable to say, but in one of my own specimens of 

 a Pharetronid fi-om Schluderbach, the spherulitic structure is very clearly marked, and 

 the systems of radiating lines are traversed by ramifying markings resembling those 

 figured b)' Steinmann, and certainly not the remains of spicules. They precisely resemble 

 the borings which are found in shells and corals at the present day, made by an 

 organism sometimes referred to the genus Achlya^. The conclusion, however, that the 

 presence of these borings is evidence that the skeleton which they penetrate has retained 

 the same structure that it had when the borings were made seems to me to be quite 

 uncertain. 



On the view that the spherulitic structure is the result of the recrystallization 

 of the lime, replacing the original spicular structure, it is not to be supposed that 

 the S3'stems of crystalline fibres have mechanically pushed their way from the centres 

 through the trabeculae of the skeleton to the destruction of any delicate bodies that 



1 Steinmann G. " Pharetroneu-Studien." Nnuex Jahrbuch j'i'ir Mineralogie, &c. Jalirg. 1882, Bd. ii. p. 139. 

 The passage alluded to is on p. 186, "Es liegt auf der Hand..." 



^ Palaeospongiologie {Palaeontographica, Bd. XL.), p. 100. 



^ Achlya is one of the Saprolegnieae. The grounds for this view of the relationship of the organism seem 

 insecure. 



