172 PROr. ST. GEORGE MIT ART ON 



pelago, where tliey are much more abundant than in any part of 



the Atlantic." 



As to the geographical distribution of different species, 

 little can as yet be said, though doubtless when the detailed 

 information collected by the naturalists of the 'Challenger' is 

 published we shall then derive much information on the sub- 

 ject. Some species have certainly a very wide range. This is the 

 case, e. g., with ThalassicoUa and Sphcerozoitm , which have been 

 found both in tropical and extratropical seas. 



I should suspect that the exceptionally clear water of the Eed 

 Sea, devoid of impurity by reason of its riverless coasts, would be 

 a very favourable station for these animals, unless the heat should 

 be too great. But few forms have yet been seen in the North Sea, 

 yet they exist in Baffin's Sea and Davis Straits. 



As to the Mediterranean, Miiller found Acanthometrine forms 

 to predominate at Cette, and Polycystine forms at Nice, together 

 with ThalassicoUa. At St. Tropez he found all three. The dis- 

 tribution of the shells described by Ehrenberg is very extensive, 

 reaching from the North Atlantic to the South-Polar Sea. 



Their geological distribution is confined, for the most part, to the 

 Tertiary rocks. If, however, Traquairia should be a Eadiolarian, 

 they would then extend back to the Carboniferous period. As 

 to the intervening Secondary epoch, the flints of the Chalk are 

 suspected to be partly formed by a redeposition of silica from their 

 dissolved skeletons. Against this, however, Haeckel objects, be- 

 cause of their conspicuous absence amidst preserved Diatoms (so 

 much smaller) and Polythalamiau shells (so much more friable). 

 Ziltel, however, has shown that they exist in the Chalk, and there 

 are imperfect remains even in the Trias. 



Eadiolarlans, as is well known, largely contribute to form masses 

 of Tertiary rock at Oran and in Barbadoes. They are also found 

 fossil in Sicily, at ^gina in Greece, at Eichmond and Petersburg 

 in Virginia, at Piscatavey in Maryland, in Bermuda, between 

 Chile and Bolivia, near Kasan, and in the Nicobar Islands. Only 

 in the last-mentioned locality and in Barbadoes have they been 

 found in large quantities and in many «pecies (100 species in the 

 former locality and 282 in the latter) forming great masses, which 

 attain the height of 1100 feet in Barbadoes, and 2000 feet at 

 Nicobar. 



As far as yet known, the most ancient forms appear to be Poly- 

 cystine Eadiolarians — Cyrtidans, especially the Zygocyrtida, — as 

 many as 229 kinds out of the 282 found in Barbadoes being Cyr- 



