HYDROIDA. 



20I 



orange, and of irregular shape (fig. 83, A). Examined microscopically, the 

 organism in Solenopora is found to be wholly composed of radiating 



*/ 



'<t~S 



Fig. 82. — A fragment of limestone rom the Lower Carboniferous Series of the south of Scot- 

 land, largely composed oiMitcheldea.7iiagrega.ria, of the natural size. (Original.) 



capillary tubes, arranged in concentric strata. The tubes (fig. 83, d) vary 

 from T T o to 2*0 millimetre in size, and are in direct contact throughout, no 

 interstitial tissue of any kind being developed. The tubes are irregular 



Fig. 83. — A, A small specimen of Solenopora compacta, Billings, from the Ordovician rocks of 

 Saak, Esthonia, of the natural size ; b, Surface of a piece of limestone largely made up of small 

 specimens of the same, from the same locality, of the natural size ; c, Tangential section of the 

 same, enlarged about 35 times ; d, Vertical section of the same, similarly enlarged. 

 (Original.) 



in form, with thin, often undulated walls, which are not pierced by any 

 apertures or pores, but are often crossed by more or fewer transverse 



