64 AMMONITIDiE. 



this reversal of terms as objectionable, inasmuch as their excep- 

 tional use in the shells of tetrabranchiates must give rise to 

 great confusion. He uses also the word " pilse " for ribs, and 

 " genie ulffi " for the knees of the ribs. 



A clear exposition of the rcA^ersed position of the animal of 

 the tetrabranchiate, in relation to its shell, may be found in a 

 paper by Prof Owen, Zool. Froc, 955, 1818. 



Neumayr (1875) and Mojsisovics (1879, 1882) proposed sys- 

 tematic arrangements of the Ammonitidse which agree in most 

 of the main groups, but differ in details. The former dismisses 

 Prof. Hyatt's classification (limited to Liassic species) with 

 scant notice, his genera not being even enumerated, " because 

 they do not agree with natural groups." I believe that both 

 Hyatt and Neumayr would find it difficult to correlate their 

 respective genera. The attempt has been made, however, by Dr, 

 Paul Fischer in his excellent " Manuel de Conchyliologie," and 

 if the result has not been satisfactory in all cases, it is at least 

 far preferable to perplexing the student with three or more sets 

 of generic names and diagnoses by presenting the several class- 

 ifications in succession. I adopt Dr. Fischer's conscientious 

 arrangement for the present, and hope that it will, as soon as 

 may be, give place to a better grouping. 



Siphonal neck of the septa directed backwards (Retrosiphonata). 



Goniatitse. 

 Siphonal neck of the septa directed forwards (Prosiphonata). 

 No Aptychus, or corneous Anaptychus of one plate (Anap- 

 tychidea \ 



First saddle of the suture broad (Latisellata). ArcestiB, 

 Tropitse, Ceratitse, Clydonitae. 



First saddle narrow (Angustisellata). Pinacocerae, Amal- 

 these, Ammonitse, Lj^tocerse, Phjdlocerse. 

 Aptychus calcareous, double, or of two valves soldered to- 

 gether (Apt3^chidea). Harpocerse, Stephanocerse, 



As Dr. Fischer regards the Ammonitidae as an order, he has 

 given the family termination to the above names : they will 

 here be considered as group-names for assemblages of genera of 

 the family Ammonitid?e. The position and appearance of the 

 lobes and saddles of the septal sutures are represented and ex- 

 plained, ii, 22, 28 ; xxxii, 31 ; xxxiii, 41 ; xxxiv, 51, 52 ; xxxv, 72. 



Goniatitae. 

 Shell nautiloid ; siphonal neck of the septa directed back- 

 wards. Siphon excentric. First whorls of the spire sometimes 

 contiguous and covered as in Ammonites, sometimes not in con- 

 tact, as in those of Spirula. 



