68 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



It is true that phylogenetic trees, those "intellectual weeds" as 

 Professor Sollas once wittily clubbed them, are rather out of fashion. 

 Still, they serve a very useful purpose, bringing before the mind by the 

 agency of the eye, more rapidly than whole pages of text can do, 

 the opinion of the moment on the interrelationships of the members of 

 the groups they deal with. 



Next let us take the scheme thus set forth and apply it to the 

 geological record after the manner shown in the succeeding diagram 

 and examine the result. In this diagram the spaces allotted to the 

 several geological formations approximate their relative thicknesses, 

 and consequently to some extent, also, show their relative periods of 

 duration. 



Beginning with the highest molluscan group — the Cephalopoda — 

 we find that the most archaic forms, the Tetrabranchia, of 'which 

 Nautilus is alone the living representative, are met with as early 

 as the Upper Cambrian, testified by seven species belonging to the 

 genera Orthoceras, Endoceran, Biloceras, Actinoceras (?), and Cyrtoceras 

 {17, pt. i). Continuing up into the Oidovician, the lower beds yield 

 a transitional form, Bactrites {17, pt. iii), which, passing up to the 

 Carboniferous, connects on through the Devonian Goniatites ( Clymenia, 

 etc.) with the Ammonites that flourished in the Jurassic and died out 

 in the Cretaceous. 



A long interval elapsed between the appearance of the Tetrabranchs 

 and the arrival of the Dibranchs. From the Lower Muschelkalk 

 (Middle Trias) near Sondershausen, Pieard (4<?, p. 308) describes 

 a form, Cmnpylosepia triassica, which he considers to be an important 

 transitional link between the Belemnites and the Sepias. Mr. Crick, 

 however, with characteristic caution, is not prepared, owing to the 

 obscure condition of the fossil, to endorse this opinion. At anj' rate, 

 we have here a forerunner of the Decapods, of which the more primitive 

 branch, the CEgopsida, are definitely represented in the Upper Trias 

 by Atractites and Aulacoceras, and the higher, Myopsida, in the 

 succeeding Jurassic by Geoteuthis, Beloteuthis, and Teuthopsis. The 

 highest cephalopod group of all, the Octopoda, made their appearance 

 in the Upper Cretaceous, with the oldest and only known fossil example, 

 Pal(Boctopus Neivholdi, H. Woodward {58). 



So far, then, as the Cephalopoda are concerned the order of their 

 appearance in geological time corresponds almost exactly with the 

 phylogenetic scheme. 



An interesting fossil group, the Conularida, may be taken next if 

 only because a cephalopod affinity has been claimed for it. At one 

 time they were relegated to the Pteropoda, and Barrande {1, p. 134), 

 Matthew {30, p. 104), and others even identified certain lower 

 Palaeozoic forms with the existing genera Styliola and Creseis. The 

 discovery of the morphologists, however, that the Pteropoda were in 

 reality highly developed and specialized Opisthobranchs has in this 

 instance been taken to heart by their palaeontological confreres, and it 

 is now generally recognized that Pteropods are not met with in earlier 

 rocks than those of the Tertiary period. 



The latest monographer of the Conularida, Miss Ida L. Slater (47), 



