208 Agassi^ Contributions to the 



Among Acephala there is not any more uncertainty respecting 

 the relative rank of their living representatives, than among 

 Echinoderms. Every zoologist acknowledges the inferiority of 

 the BLyozoa and the Brachiopods when compared with the 

 Lamcllibranchiata, and among these the inferiority of the Mono- 

 myaria in comparison with the Dimyaria would hardly be denied. 

 Now if any fact is well established in Palaeontology, it is the 

 earlier appearance and prevalence of Bryozoa and Brachiopods 

 in the oldest geological formations, and their extraordinary deve- 

 lopment 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 different 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 palaeon- 

 tologist is aware how imperfectly their remains have been inves- 

 tigated 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 Sipho- 

 nostomata to have appeared later than the Holostomata, and this 

 exhibits already a general coincidence between their succession 

 in 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 every- 

 body 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 formations, and the later appearance, about 

 the middle geological ages, of representatives 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 ; but the Crustacea exhibit, again, the most strik- 

 ing coincidence. Without entering into details, it appears from 

 the classification of Milne-Edwards that Decapods, Stomapods, 

 Amphipods, and Isopods constitute the higher orders, while 

 Branchiopods, Entomostraca, Trilobites, and the parasitic types, 

 constitute, with Limulus, the lower orders of this class. In the 

 classification of Dana, his first type embraces Decapods and 

 Stomapods, the second Amphipods and Isopods, the third Ento- 

 mostraca, including Branchiopods, the fourth Cirripedia, and the 



