KADIOLARIA. 



131 



\ 



earth," as this deposit is generally called, is thus a Tertiary 

 representative of the " Eadiolarian ooze," which has been 

 shown by Sir Wyville Thomson to cover large areas of the 

 ocean-bed, up to the enormous depth of 4500 fathoms, and 

 which is likewise principally made up of the shells of Poly- 

 cystines. As is the case, however, with the Globigerince, the 

 Polycystines are principally surface -forms, inhabiting the 

 open ocean, so that a deposit formed of their shells can in 

 no way be regarded as indicating necessarily the depth of 

 the sea in which it was laid down. 



There are many known recent types of Polycystina which 

 have not yet been detected in a fossil state ; and the prin- 

 cipal fossil forms are the following : One of the simplest, 



Fig. 31. Types of Polycystina. a, Podocyrtis Schomburgki ; 6, Dictyomitra Mongolfieri; c, 

 Haliomma dixiphos; d, Dictyocha Messanensis; e, Eucyrtidium elegans; f, Lychnocanium 

 lucerna. d is living, and is after Hseckel ; the remaining are Tertiary, and are after Ehre'n- 

 berg. All the figures are greatly enlarged. 



and it is also the most ancient, of the fossil types is Cozno- 

 spJwcra (sometimes referred to the Thalassicollicla), which is 

 found in the Jurassic and Cretaceous, and has survived to 

 the present day. Of the typical Polycystines, Haliomma 

 (fig. 31, c), Heliodiscus, Actinomma (fig. 30, 6), and Didymo- 

 cyrtis, represent those forms in which the skeleton consists of 

 two, three, or more porous spherical shells, included concen- 



