SUBCLASS II 



DIBRANCHIATA 



677 



develojjmeiit, and sucli a great mass of evidence in favour of the theory of evolution 

 as liave the Amnionites, Indeed, in this group, on accoiint of tlie nncommon thinness 

 of tlie shell, internal nioulds are of as mucli importance froni tlie standpoint of precise 

 identification as those specimens whicli have the shell perfectly preserved. 



Tlie first attempt to study a large number of species of Ammonites in tlieir 

 genetic relationships was made by Waagen in the series of Oppelia supradiata. 

 Similar attempts were made by Neumayr in the Phylloceratidae, Perisphinctinae, etc. ; 

 by Hyatt in the Arietitinae ; and with especial minuteness by Leopold Würtenberger 

 in the Jurassic groiips Äspidocei-as, Siinoceras, Waagenia, Peltoceras, Periaphinctes and 

 Stephanoceras. Also Mojsisovics, Uhlig, Hang, Douville, Frech, Diener, Pompeckj and 

 others have paid special attention to the genetic relationships of the various groups of 

 Ammonites. All these authors come to the conclusion that in the Ammonites there 

 are numeroiis genetic series of which the development may be followed step by step in 

 the species that occur in the various successive strata. 



In the last few years great progress has been made in the study of the Ammonite 

 launas, especially of the later Paleozoic and Triassic horizons ; also in the most various 

 divisions of the many branched family tree of the Ammonites, much light has been 

 thrown upon the genetic relationships of numerous genera and families. But in 

 spite of this it is not yet possible to give a graphic representation of the development 

 and kinship of the Ammonoidea that is true of the whole group, and beyond suspicion 

 in any of its parts. However, in the above diagram a tentative effort is made in this 

 direction, and in this scheme the probable relationships of the Paleozoic to the 

 earlier Mesozoic genera are indicated in the light of ^ 



the present Status of our knowledge. ' 



[The foregoing chapter on Ammonoidea has been revised 

 for the present work by Professor James Perrin Smith, of 

 Leland Stanford Junior University, California. — Editor.] 



Subclass 2. DIBRANCHIATA Owen. 



Cephalopods with only two arborescent gills in the 

 mantle-cavity : provided round the mouth with eight 

 or ten arms hearing suckers or hooks, two of them 

 (when ten in all are present) heing often developed 

 into long tentacles. Funnel closed ; ink-sac usually 

 present Shell internal, or if external, it is not 

 chamhered ; in many forms entirely wanting. 



The body of the Dibranchiates or Cuttle-fishes 

 is elongated, cylindrical or sac-shaped, and fre- 

 quently provided with two lateral fin-like append- 

 ages. The anterior cephalic region gives ofF a 

 circlet of eight or ten powerful, muscular arms, 

 the inner sides of which are armed with suckers 

 (acetahula), or a double row of hooks, and assist 

 in swimming or creeping, and also serve for the 

 capfcure of prey. The Sepioidea have two of 

 their ten arms developed into very long tentacles 

 which bear hooks or suckers only at their 

 thickened extremities (Fig. 1313). The lower surface of the suckers is disk- 

 or cup-shaped, perforated in the middle, and occupied by numerous radially 



Fig. 1313. 



Enoploteuthisleptura. Recent; 

 Pacific Ocean. A, Ventral aspect. 

 B, Internal shell or "pen." 



