672 



MOLLUSCA 



PHYLUM VI 



In this familyi probably beloiig several geiiera comnionly classed witli Oxynoticeras 

 or Amaltheus, as follows :— Garnieria Sayii. ; Lenticeras Gehr. ; Eulophoceras Hyatt. 



Family 3L Prionotropidae Zittel. 

 Form evolute. discoidal, laterally compressed. Flanhs with strong, simple or 

 dichotomous ribs that form one or more rows of hnots on the sides, and one on the 

 ventral Shoulders. Venter with sträng median keel, either smooth or hrolen up mto a 



Fi(!. 1311. 



Schloenbadda varians (Sowb,)- Cenomanian 

 Quedlinburg, Saxony. 



Fig. 1312. 



SchloenbacMa cristata (Deluc), 

 Lower Cretaceous. 



roiv of hnots. Septa only moderately digitate. External and first lateral saddle hroad, 

 lateral lohes hifid, only one auxiliary lohe present. 



This groiip is commonly siipposed to have been derived from the Amaltheidae, 

 but proofs of the connection are lacking. 



SchloenbacMa Neumayr (Figs. 1311, 1312); Hystatoceras Hyatt; Barroisiceras 

 Gross. ; Mortoniceras Meek ; Peroniceras Gross. ; Prionotropis Meek. Cretaceous. 



Range and Distribution of the Ammonoidea. 



The Ammonoids are more than twice as rieh in forms as the Nautiloids. While 

 of the latter aboiit 2500 species have been described, the number of Ammonoids has 

 reached far beyond 5000 species. These are without exception extinct, and are 

 especially characteristic of the Mesozoic era. 



Although Ammonites are unknown later than the Cretaceous period, nevertheless 

 this group must be regarded on the whole as the younger brauch of the stock of 

 Tetrabranchiates. After the Nautiloids had passed their culmination, the Goniatites 

 and Clymenias appeared as the oldest representatives of the Ammonoids. The time 

 ränge of the Clymenias is limited to a short epoch in the Upper Devonian ; the 

 Goniatites appeared first in the Upper Silurian (Kellerwald), developed a great 

 variety of forms in the Devonian, and continue until the close of the Paleozoic era. 



Until a few years ago it was belle ved that only Goniatites and Clymenias occurred 

 in the Paleozoic deposits. The discovery of genuine Ammonites in the Permian of 

 the Salt Range of India, in the Ural Mountains, in Texas, in the Fusulina limestone 

 of Sicily, etc., and later the discovery of primitive Ammonites in the Goal Measures 

 of Texas and in the Lower Carboniferous of the Mississippi Valley, piished their ränge 



^ DonvilU, IL, Evolution et Classification des Pulchellides. Bull. See. Geol. France, 1911, vol. xi. 



