clxxiv THE VOYAGE OF H.M.S. CHALLENGER. 



are often found fossil. Of the two other legions those families which possess no skeleton 

 are of course excluded ; the Nassellida among the Nassellaeia, and the ThalassicoUida 

 and Collozoida among the Spumellaria. Thus of the 85 known families there remain 

 scarcely 55 of which the skeletons maybe expected in the fossil state; and of these 

 scarcely half have been actually observed in this condition. Of the 20 orders of this 

 class enumerated in § 155, the following 9 may be, for pateoutological and geological 

 purposes, completely excluded :—( A) The 4 orders of Acantharia (1, Actinelida; 

 2, Acanthonida; 3, Sphserophrac ta ; 4, Pr unophr act a); (B) 3 orders of 

 Ph^odaeia (5, Pha3 0sph£eria; 6, Pha^ogromia; 7, P ha3 conchia); (C) 1 

 order of Nassellaeia (8, N a s s o i d e a) ; (D) 1 order of Spumellaria (9, € o 1 1 o i d e a). 

 From a geological point of view the following 6 orders, although occasionally found fossil, 

 are of quite subordinate importance: — (A) Among the Spumellaria (10, B el o idea, 

 and 11, Larcoidea); (B) among the Nassellaria (12, Plectoidea; 13, Ste- 

 phoidea; 14, Botryodea); (C) among the Ph^odaria (15, the P haeocystina). 

 On the other hand the following 5 orders, which are the main constituents of Radiolarian 

 rocks, are of pre-eminent geological importance : — (A) Among the Spumellaria 

 (16, Sphseroidea; 17, Prunoidea; 18, Discoidea); (B) among the Nassel- 

 laria (19, Spyroidea, and 20, Cyrtoidea). The numerical relation in which the 

 different families of these ordei-s appear in the Radiolarian formations may be seen on 

 consulting § 157. 



250. Fossil and Recent Species. — The fact that there are many Radiolaria living at 

 the present day, whose shells are found fossil in Tertiary rocks, is of great phylogenetic 

 and geological significance. This appeared to be the case even from the older observa- 

 tions upon the Polyeystina of the Barbados marl (see note A), but more recent and 

 extensive observations both upon these and upon the Miocene Radiolaria of Sicily, have 

 shown that the number of these " living fossil " forms is much greater than was pre- 

 viously supposed (see note B). Among the Miocene Radiolaria numerous species, both of 

 Spumellaria (especially Sphseroidea and D i s c o i d e a) and of Nassellaeia 

 (especially Spyroidea and Cyrtoidea) are not to be distinguished from the corre- 

 sponding still living forms (see notes C, D). On the other hand, those genera, which are 

 rich both in species and indi\dduals (recent as well as fossil), present continuous series of 

 forms which lead gradually and uninterruptedly from old Tertiary species to others still 

 living, which are specifically indistinguishable from them. These interesting morpho- 

 logical facts are capable of direct phylogenetic application, and furnish valuable proofs 

 of the truth of the theory of descent. 



A. Ehrenberg, in his list of fossil Polyeystina (L. N. 25, pp. G4-85, 1875), records 325 species 

 of which 26 are still living. 



