DIBRANCHIATE CEPHALOPODA. 



501 



growth of the shell comprise Ancyloceras and Helioceras, which range 

 from the Inferior Oolite to the Chalk ; Crioceras and Toxoceras ex- 

 tend from Neocomian to Upper Greensand ; Hamites, Ptychoceras, and 

 Baculites range from the Xeocomian to the Chalk, while Turrilites 

 ranges from the Gault to the Chalk, in which it is most developed. 



The Tetrabranchiata are therefore of great stratigraphical value ; 

 and the large number of fossil species invests them with importance, 

 for it would be easy to make a Table of strata in which each marine 

 bed was distinguished by well-characterised and easily-recognised 

 Cephalopods. 



The Dibranchiata are especially characteristic of existing seas; 

 they are naked Cephalopods, which have an internal shell or pen, an 

 inkbag about the mouth, and only eight or ten arms, which are pro- 

 vided with suckers or horny hooks. No representative of the group 

 has been found in the Primary rocks ; it begins with the Lias, where 

 the living genus Loligo first appears. Teudopsis, Belotheuthis, Geo- 

 theusis, and Plesiotheusis are all Liassic genera which range into the 

 Lower Oolites. Leptotheuthis is found in the Oxford clay and at 

 Solenhofen. Ommastrephes ranges from the Oxford clay to existing 

 seas ; and Enoplotheuthis from the Oolite to existing seas. 



The most interesting genera of Dibranchiates are the Belemnites, 

 which are almost entirely Secondary. Belemnites range from the Lias 

 to the Chalk, though a belemnite-like fossil has been found 

 in the Tertiary of South Australia. Xipoteuthis is a genus 

 with a fusiform phragmocone found in the Lias. Belem- 

 noteutliis characterises the Oxford and Kimmeridge Clays. 

 Conoteuthis ranges from the Neocomian to the Upper Green- 

 sand. Belemnitella is limited to the Upper Greensand and 

 Chalk. Sepia has survived from the Oxford clay ; and the 

 allied Belemnosis and Beloptera are found in the London 

 Clay and Bracklesham beds. 



Fishes. Although existing fishes are divided into four 

 sub-classes, Palseichthyes, Teleostei, Cyclostomata, and Lep- 

 tocardii, only the two former can be recognised in a fossil 

 state. The Palaeichthyes, which comprise the majority of 

 the fishes found in the older rocks, are divided into five 

 orders, the Dipnoi or mud fishes, the Ganoidei, the Holo- 

 cephala or Chimseras, the Plagiostomata or sharks and 

 rays, and the Chondrostei or sturgeons. Palaeichthyes have 

 a spiral valve to the intestine. Dr. Glinther remarks that 

 they stand to the Teleostei in the same relation as the 

 Marsupialia to the Placentalia among mammals. This view 

 is perhaps favoured by Alexander Agassiz's embryologicai 

 work, 1 which has shown that Teleostean fishes in an early 

 stage of development have the tail symmetrical, without 

 either the homocercal or the heterocercal modification (fig. 136), such as 

 is seen in most of the Ganoids of the Old Red Sandstone and in the 



1 Young, " Stages of some Osseous Fishes," Proc. Americ. Acad., vol. xiii., xiv. , 

 xvii. 



Fig. 135. 



Belernni- 



tella. 



