Chap. I. SUCCESSION AND STANDING OF ANIMALS. 109 



period to the present da}', coincides also Avith the gradation of their subdivisions, 

 as determined by their structure; and it Avas not until the present period, that the 

 highest Echiiioderms, the Ilolothurioids, have assumed a prominent position in their 

 class. 



Among Acephala there is not any more uncertainty respecting the relative rank 

 of theii- living representatives, than among Echinoderms. Every zoologist acknowl- 

 edges tlio inferiority of the Bryozoa and the Brachiopods^ -when compared -with the 

 T^amellibranchiata, and among these the inferiority of the Monomyaria in compari- 

 son with the DimjMria would hardly be denied. Now if any fact is well established 

 in Palaeontology, it is the earlier appearance and prevalence of Brj^ozoa and Bra- 

 chiopods in the oldest geological formations, and their extraordinary development 

 for a long succession of ages, until Lamellibranchiata assume the ascendency which 

 they maintain to the fullest extent at present. A closer comparison of the differ- 

 ent families of these orders might further show how close this correspondence is 

 through all ages. 



Of Gasteropoda I have nothing special to say, as every palaeontologist is aware 

 how imperfectly their remains have been investigated in comparison with what has 

 been done for the fossils of other classes. Yet the Pulmonata are known to be 

 of more recent origin than the Branchifera, and among these the Siphonostomata 

 to have appeared later than the Holostomata, and this exhibits already a general 

 coincidence between their succession m time and their respective rank. 



Our present knowledge of the anatomy of the Nautilus, for which science is 

 indebted to the skill of Owen,^ may satisfy everybody that among Cephalopods the 

 Tetrabranchiata are inferior to the Dibranchiata ; and it is not too much to .say, 

 that one of the first points a collector of fossils may ascertain for himself, is the 

 exclusive prevalence of the representatives of the first of these types in the oldest 

 fijrmations, and the later appearance, about the middle geological ages, of represent- 

 atives of the other type, which at present is the most widely distributed. 



Of Worms, nothing can be said of importance with reference to our inquiry ; 



' OKiur.xy. (A. i>'.) Bryozoire.^, Aim. Sc. Xat.. ."ie Sioilia?, eorumque Ilistoria ct Anafomin, Parma-, 



ser. 18.')1, vol. 10, p. 20-2.— Cuvier, (G.,) Mt'moire 17'.tl-9S, 2 vols. fol. fig.. TOiitinuwl l.y D.'ll." Cliiaje. 

 sur ranimal de la Lingule, Ann. Mus. I., p. G9. tig. — - Owen, (R.,) Memoir on the Pearly Nautilus, 



YoGT, (C.) Anatoniie der Lingula anatina. N. Mem. London, 1832, 4to. fig. — Valescienxes. (A..) Xou- 



Soc. IIclv. 1843. VII., 4to. fig. — Owr.x, (K.,) On the velles Kccherches anatoini(iues sur le Xautile. C. 11., 



Anatomy of the Bratliiopoda, Trans. Zool. Soc., I. Paris, 1841, 4to. — Cuvier, (G..) !Memoires pour 



4to., p. 1 4."), fig. — On the Anatomy of the Terebratula, servir a ITIistoire ct .'i TAnatoinie des Mollusques, 



l.S.-)3, 4to. fig. (PaUeont. Soc.)— Brcn, (L. v..) Ucber Paris, 1817, 4to. fig.— Edwards, (II. M..) Quatke- 



Tcnhratcln. t\. a., p. 97. — Davidson. (Til.) ^lonogr. faces, (Ar. de.) et Blanciiard, (E.m.) Voyage en 



ck-.. (1- a., p. 97. — I'm. I (Xav.,) Testacca utriusque Sicile, Paris, 3 vols. 4t(). fig. (without date.) 



