600 SCIENTIFIC RECORD FOR 1882. 



years ago urged that they weie ordinally distiuct from both the Tetra- 

 branchiates aud Dibraiichiates, aud distinguished them as the order ot 

 Aminouoids, aud was followed therein by Professor Hyatt, and M. 

 Munier-Chalmas,in 1874, even considered them to be nearer the Dibraii- 

 chiates than to the Tetrabranchiates. Kecently M. Fischer has acceded 

 to the view that they represent au order (" Ammouea "). As to the fur- 

 ther subdivision of the class, the Dibranchiates 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 elongated 

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

 out the corresi)onding arms have a roundish sacciform body and are 

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



The families of Ammonoids. 



The "order Ammonea" or Ammouoids is primarily subdivided by 

 Fischer into (1) the Retrosiphonata, in which the partitions are iufundi- 

 bulate backwards around the siphon, aud (2) the Prosii^honata, 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; thej' 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, aud {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. Petrosiphonata {Goniatitidce). 



B. Prosiplu)uata a. Anaptychidea. Section 1. Latisellata. — {Arces- 

 tidcCj Trojntidcv, Ceratitidcv, Glydoiiitidcc). Section 2. Angustisellata. — 

 {PinacoceratidcB, AmaltheidWj Ammonitidce^ Lytoceratidce). h. Aptychi- 

 dea {Harpocerotidfv, Steplianoceratidcc). 



Some American palieontologists, especially Professor Hyatt and the 

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

 for other reasons. 



Fumllks of Tetrabranchiates. 



M. Fischer has admitted only three families of Tetrabranchiates in 

 his recent classification of the Cei)halopods — the prosiphonate Notho- 

 ceratids and the retrosii)honate iSfautilids 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 {Sothoceratidw). 



B. Retrosiphonata [Xautilidw, Ascoceratidce). 



