200 



CCELENTERATA. 



from the Chalk which have been placed in the genus Porosphcera 

 have often been regarded as related to Parkeria, but their minute 

 structure would rather support their being placed among the 

 Sponges. 



In connection with the preceding, a few words maybe said with regard 

 to the singular genera Mitcheldeauia and Solenopora, which are of 

 doubtful affinities, though the balance of evidence is in favour of referring 

 them to the Hydrozoa. The genus Mitcheldeania comprises small 



-Vertical section across the centre of a specimen of Parkeria, enlarged twice. 

 p, One of the radial pillars ; c, One of the chamberlets. (Original.) 



spheroidal fossils rarely reaching half an inch in diameter, which some- 

 times occur in such vast quantities as to give rise to actual beds of the 

 Carboniferous Limestone (fig. 82). When examined microscopically, the 

 skeleton of M itcheldea?iia is found to be formed of radiating capillary 

 tubes, which are disposed in concentric strata, and have a diameter of 

 from 3*2 to ^5 of a millimetre. These tubes have porous walls, and are 

 united by a still more minutely tubulated tissue ; but the structure of the 

 organism is too complex to be advantageously discussed here. 



The genus Sole?iopora comprises calcareous organisms of varying 

 size, usually ranging from the dimensions of a pea up to those of an 



