importance:--(A) Among the SPUMELLARIA (16, #Sphæroidea#; 17, #Prunoidea#; 18, #Discoidea#); (B) among the NASSELLARIA (19, #Spyroidea#, and 20, #Cyrtoidea#). The numerical relation in which the different families of these orders 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 observations upon the Polycystina 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 previously supposed (see note B). Among the Miocene Radiolaria numerous species, both of SPUMELLARIA (especially #Sphæroidea# and #Discoidea#) and of NASSELLARIA (especially #Spyroidea# and #Cyrtoidea#) are not to be distinguished from the corresponding still living forms (see notes C, D). On the other hand, those genera, which are rich both in species and individuals (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 morphological 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 Polycystina (L. N. 25, pp. 64-85, 1875), records 325 species of which 26 are still living. {clxxv}B. Stöhr, in his list of Miocene Radiolaria from Grotte (L. N. 35, p. 84, 1880), records 118 species, of which 29 are still living. C. Teuscher, who at my request has made a large number of comparative measurements and drawings, both of fossil and living Radiolaria, comes to the conclusion that numerous SPUMELLARIA and NASSELLARIA from Barbados are to-day extant and unchanged in the Radiolarian ooze of the deep Pacific Ocean (compare § 242A, and p. 1760, Note). D. From the comparative investigations, which I have made during the last ten years into the recent deep-sea Radiolaria of the Challenger collection and the Miocene Polycystina of Barbados, it appears that about a quarter of the latter are identical with living species of the former. {clxxvi}BIBLIOGRAPHICAL SECTION. CHAPTER XI.--LITERATURE AND HISTORY. 251. _List of Publications from 1834 to 1884_:-- _Note._--In the text the references to the following publications are indicated by the letters L. N. 1. 1834. MEYEN, F., Palmellaria (Physematium, Sphærozoum), in Beiträge zur Zoologie, gesammelt auf einer Reise um die Erde. _Nova Acta Acad. Cæs. Leop.-Carol._, vol. xvi., Suppl., p. 160, Taf. xxviii. figs. 1-7.