90 Prof. P. M. Duncan on Ujjper Silurian 



have the general and some of the special characters of those 

 Lithistid Spongida which belong to the Tetracladina. 



But the fossilization of the skeletal parts of the new form is 

 not that which is characteristic of mechanical infilling after 

 outgoing of a former mineral — such, for instance, as is seen 

 in calcareous replacement of siliceous spicules. The calcareous 

 mineral of the skeleton is not in distinct crystals, and cleavage- 

 planes are rare ; on the contrary, the mineralization re- 

 sembles that of fossils which were originally of carbonate of 

 lime. 



There is a point of some interest which offers some evi- 

 dence that the original skeleton was not siliceous. In the 

 midst of the long canals, in their interspaces, and passing 

 over the skeletal parts in close proximity are many relics of a 

 large form of PalaiaclJya penetrans, Dune. ; and in sections 

 the passage of the tubes of the parasite through and along the 

 inside of the spicules can be seen. Usually the tube is 

 crammed with the spherical spores ; and they frequently 

 extend beyond it and collect in masses. In one instance 

 they crowd a spicule. The tubes and spores are, as in the 

 specimens described in a former number of the ' Quarterly 

 Journal of the Geological Society,' carbonized. 



It does not appear to me to be likely that these parasitical 

 plants penetrated after the calcareous fossilization of the inter- 

 stices was completed ; they must be regarded as having grown 

 at the expense of the organic matter of the spicules during the 

 lifetime of the organism. Moreover it must be conceded, 

 from the knowledge we have of the physiology of the Achlya 

 group, that it is not probable that they could penetrate and 

 live in silica. 



These little spheroidal fossils are, then, of considerable 

 interest ; and the more they are critically and carefully exa- 

 mined with all the appliances of the microscope, the more do 

 they resemble the Spongida. Their texture is not that of 

 the perforate coral ; and they have no accurate and minute 

 resemblance to the Tabulata ; but they are most suggestive 

 from their transitional appearance. 



If all the modern Lithistids were siliceous, there must have 

 been a former mimetic and calcareous group of Spongida. Or, 

 as the Lithistids appear to have been rare in the earlier fossili- 

 ferous rocks, and Anloco'pium of the Silurian is the first known, 

 it is possible that a group of Calcareo-Spongida lived contempo- 

 raneously and became extinct or merged into a higher form 

 as the parent of Zoantharia Perforata. I have named the 

 fossils, after their discoverer and their shape, Hindia sjjJicv- 

 roidalis. " 



