SUBCLASS II 



DIBRANCHIATA 



677 



development, and such a great mass of evidence in favour of tlie tlieory of evolution 

 as have tlie Ammonites. Indeed, in this group, on account of the uncommon thinness 

 of the shell, internal moulds are of as much importance from the standpoint of precise 

 identification as those sjiecimens which have the shell perfectly preserved. 



The first attempt to study a large number of species of Ammonites in their 

 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 Wiirtenberger 

 in the Jurassic groups Aspidocei-as, Simoceras, Waagenia, Peltoeeras, Perisphindes and 

 Stephanoceras. Also Mojsisovics, Uhlig, Hang, Douville, Freeh, 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 numerous 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 

 faunas, 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 ujion the genetic relationships of nr;merous 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 efl:ort 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. 



Cefhalopods with only two. arhoi'escent 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) being often developed 

 into long tentacles. Funnel closed; ink-sac usually 

 present. Shell internal, or if external, it is not 

 chambered ; 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 

 capture 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 



Pig. 1313, 



Enoploteuthis leptura. Recent; 

 Pacific Ocean. A, Ventral aspect. 

 B, Internal shell or "pen." 



