122 On the Microscopical Siliceous Polycystina of Barbados, 



liommatina ; likewise the posterior apertures of the shields or 

 shells of several forms which appear in elegantly-shaped denti- 

 culated rings ; or the lateral rays, spines and feet occur in a free 

 spinulate form ; and the broken trellis-like portions of their bodies 

 might be compared to siliceous nets. In some of the specimens 

 of rock where the forms are well-preserved, the origin of the 

 fragments is easily recognized ; this is unfortunately seldom the 

 case ; and Ehrenberg observes, that under such circumstances it 

 is requisite to draw a conclusion from such fragments as are re- 

 cognizable, in a similar manner as in a geological point of view 

 sharks' teeth must serve to determine the species from which 

 they came. Hence Professor Ehrenberg has divided the Geolithia 

 into the following groups, which for the sake of analogy he terms 

 only provisionally genera : — 



Stelliform siliceous fragments Actinolithis. 



Net-like „ „ Dictyolithis. 



Annular ,, „ Stephanolithis. 



Tabular „ ,, Placolithis. 



Staff-like ,, „ Rhabdolithis. 



Nuciform cellular fragments (Polycystine nuclei) ... Caryolithis. 



Cephalotic and rostrate fragments (Polycystine beaks) Cephalolithis. 



In some instances the whole geological specimen from Barba- 

 dos is composed of such fragments, in which case it becomes dif- 

 ficult to determine the true species ; the genera however are easily 

 defined. In the atmospheric dust borne along by the storm, and 

 in the dust from the craters of volcanos, it will henceforth be 

 easier to recognise the siliceous forms of Geolithia, and to com- 

 pare and determine them with as much certainty for the purpose 

 of geological deductions as the spines of Echini, the teeth of 

 fishes, scales and bones of various kinds. 



Ehrenberg has determined twenty-seven species of Geolithia, 

 several of which bear no relation to any of the 282 species of 

 Polycystina from Barbados, but possess nevertheless well-defined 

 characteristic forms. If any of these fragments should hereafter 

 be discovered in atmospheric dust, we should be authorized to 

 consider Barbados as its source. The remarkable genera Actinoli- 

 this and Placolithis are quite unknown with respect to their origin. 



" If an unknown net-like or reticulated fragment is hereafter 

 named a Dictyolithis of such a form and size, it will be a more 

 appropriate and shorter description, and less subject to error, 

 than if described as Podocyrtidis or Spongice, or Eucyrtidii frag- 

 mentum, or by any other name. Where it is not requisite to notice 

 mere fragments, no person would reasonably enumerate them." 



Professor Ehrenberg has next directed his attention to the 

 partly organic, partly morpholithic calcareous ingredients, which, 

 besides the siliceous, are contained in the Barbados marl. The 

 organic parts consist of a small number of Polythalamia : the 



