TBE DESCENT OE THE OCTOPODA, 



A CONTRIBUTION TO A MORE NATURAL CLASSIFICATION. 

 BY E. H. L. SCHWARZ, A.R.C.S. 



The Cephalopoda are usually now divided into the Tetrabran- 

 chiata and the Eibranchiata, terms which, though relying on a 

 character, viz. the number of gills, which must always remain 

 hypothetical in the vast majority of forms, nevertheless, till lately, 

 divided this group of Mollusca into two divisions, apparently 

 separated by a great gap. 



The former group, except for a very few (3 or 5) species of 

 Nautilus, is extinct, and includes the animals of the camerated shells. 

 These are structures which, beginning from an embryonic protoconch, 

 are built up of a series of gradually enlarging chambers separate one 

 from another, except for a tube running uninterruptedly through 

 the whole, called the Siphuncle, which, according to Zittel,* 1 ) is the 

 remains of the visceral sac drawn out and now functionary as a 

 conveyor of nutriment to the distal parts of the shelK 2 ) In the 

 Ammonoidea the protoconch is inflated and calcified, and is usually 

 to be seen in carefully removing the matrix from the centre of well 

 preserved specimens : in the Nautiloidea on the other hand, it 

 appears to have been membranous, for no corresponding structure is 

 found except in rare cases of Orthoceras,@) but we infer that it must 

 have once been present by the occurrence of a scar or cicatrix on 

 the apex of the shell, showing the place where the embryonic shell 

 opened into the first true chamber. ( 4 ) Besides this distinction in the 

 protoconch, the Ammonoidea are distinguished from the Nautiloidea 

 by the complex folding of the wall or Septum separating two adjacent 

 chambers, and which forms the suture line on the exterior, being 

 always bent away from the mouth on the external (ventral) side, 

 forming a median lobe ; and also by the siphuncle being placed on 

 the external margin of the shell, a character however which is broken 

 in Glymenia, which has the siphuncle internal. 



The Nautiloidea are first met with in the lower rocks of the 

 Baltic Cambrian, occurring in a sandstone round St. Petersburg, 

 together with Olenellus, Cystideans, &c.,( 5 ) in the form of a small 

 Orthoceras, (Volborthella). In the Ordovician, a great number of 



(1). This has recently received fresh support from the work of Dr. G. Holm on 

 Endoceras. Dames and Kayser's Palceont. Abhandl. Vol. Ill, 1885. 



(2). Edwards and Wood, Eocene Mollusca, Pal. Soc. Moil., 1849, p. 12. 



(3). Hyatt, Arietidai, Mem. Mus. Comp. Zool., Harvard, 1889, p. 10. 



(4). Hyatt, Embryology, Foss. Ceph., Bull. Mus. Comp. Zool., Cambridge, 

 Vol. Ill, 1852. 



(5). Schmidt, Quart. Journ. Gcol. Soc, Vol. XXXVIII, p. 516. 



