600 SCIENTIFIC RECORD FOR 1882. 



years ago urged that they were ordinally distinct from both the Tetra- 

 branchiates and Dibrauchiates, and distinguished them as the order of 

 Aramonoids, and was followed therein by ProSessor Ilyatt, and M. 

 Munier-ChalraaSjin 1874, even considered them to be nearer the Dibran- 

 chiates than to the Tetrabranchiates. Eecently M, Fischer has acceded 

 to the view that they represent an order ("Ammonea "). As to the fur- 

 ther subdivision of the class, the Dibrauchiates are with one accord 

 divided into two groups (generally called suborders) distinguished by 

 the development or want of two unusually elongated tentacular arms 

 inside the circle of eight sessile arms common to all the order. Those 

 forms with tentacular arms (Decaceres or Decapods) have an elougated 

 body, with lateral fins, aud an internal "shell" or "gladius"; those with- 

 out the corresponding arms have a roundish sacciform body and are 

 destitute of an internal shell, and are known as Octopods. 



The families of Amnionoids. 



The "order Ammonea" or Ammonoids is primarily subdivided by 

 Fischer into (1) the Retrosii^honata, in which the partitions are infundi- 

 bulate backwards around the siphon, and (2) the Prosiphonata, whereof 

 the perisi phonal infundibulatiou is advanced forwards. The Prosipho- 

 nata are by far the most numerous, and were mostly formerly confounded 

 under the generic name Ammonites; they represent, however, a number 

 of families which have been segregated into two major groups, (a) the 

 Aptychidea, which have an "aptychus" composed of two calcareous 

 pieces free or consolidated, and (b) the Anaptychidea, which are desti- 

 tute of an aptychus, or at least of a calcareous one. The families ad- 

 mitted are the following : 



A. Eetrosiphonata {Goniatitidcc). 



B. Prosiphonata a. Anaptychidea. Section 1. Latisellata. — {Arces- 

 tidce, TropitidcCj Ceratitidcc, Clydonitidce). Section 2. Augustisellata. — 

 {Pinacoceratidce, AmaUheidce, Amnionitidce, Lytoceratidce). h. Aptychi- 

 dea [Ilarpoceratidw, Steplianoceratidce). 



Some American palaeontologists, especially Professor Hyatt and the 

 late Professor Agassiz, have admitted still more numerous families, and 

 for other reasons. 



Families of Tetrabranchiates. 



M. Fischer has admitted only three families of Tetrabranchiates in 

 his recent classification of the Cephalopods — the prosiphonate Notho- 

 ceratids and the retrosiphonate Nautilids and Ascoceratids. The Nau- 

 tilids include all these forms with the partitions perpendicular to the 

 axis of the shell, and the Ascoceratids, those whose partitions are very 

 oblique and become even subparallel to the axis. In fine, his arrange 

 ment is presented in the following manner : 



A. Prosiphonata [J^otlioceratidan). 



B. Eetrosiphonata [I^'autiUdw, Ascoceratidw). 



