416 ROSS ON EVOLUTION. 



seems insufficiently bridged by the two Genera, only, which are 

 known of a single Order, the Monotremata, the only surviving 

 Order of the Sub-class Ornithodelphia, it should be remembered 

 that every principle of analogy would lead us to anticipate, that 

 when that Island Continent shall have been well explored geologi- 

 cally, the remains of other Genera, Families, and even Orders will, 

 as in the case of the Ganoidea and Lahyrinthodontia among the 

 Ichthyopsidia, restore to us the connecting links which in Mesozoic 

 Periods gave an easy transition from the Sauropsidia to the 

 Mammalia. 



Of Molhisca, the Tetrabranchiate Cephalopods, of which 

 the Genus JSfaiitilus is the only living representative, possesses 

 some points of very special interest, as having chambered shells and 

 continually moving outward as they grow, the shells, w^hich have 

 also the very great advantage of being exceedingly well preserved 

 as fossils, present an epitome, perfect, so far as it goes, of the 

 entire life of the individual ; so that there exists a singularly well 

 preserved representation of the entire Order, — from its apparent 

 origin in the Lower Silurian to the present day, when it has almost 

 become extinct, — alike as regards the successive Species and the 

 successive phases in the development of the individual of each 

 Species. Orthoceras, of the lowest Silurian Epoch, the earliest 

 and simplest known type, had a shell in shape a straight cone, and 

 had simple concave septa. It was followed by such forms as 

 Oyrtoceras and Plwagmocei^as, with shells resembling a bent 

 cone, and with septa having shallow lateral lobes. After these 

 comes Gyroceras, in which the bending of the shell has so much 

 increased as to give it the form of a loose coil, and in which the 

 lobes have become deeper, followed by others in which the coil has 

 become close, and the latter lobes more angular, until the shell 

 has become involute and the umbilicus has been obliterated, as in 

 jSfaidiliis ziczac, of the Tertiary, and Ae living representative^ of 

 the Family. 



The x\mmonite series, in which a similar succession of forms 

 occur, are remarkable for complication of their septa and the profu- 

 sion of their ornamentation at the time of the Jurassic Period, when 

 they had the greatest number of Speciiic forms. But this is true 



