672 



MOLLUSCA 



PHYLUM VI 



In this family 1 probably belong several genera commonly classed witli Oxynoticeras 

 or Amaltheus, as follows : — Garnieria Sajai. ; Lenficeras Gehr. ; Euloplwceras Hyatt. 

 Cretaceons. 



Family 31. Prionotropidae Zittel. 



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

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

 ventral shoulders. Venter with strong median heel, either smooth or broken ^ip into a 



Fig. 1311. 



Scldoenbachia varians (Sowb.). Cenomanian ; 

 Quedliiiburg, Saxony. 



Fio. 1312. 



Schloenhachia cristata (Dehic). 

 Lower Cretaceous. 



row of knots. Septa only moderately digitate. External and first lateral saddle broad, 

 lateral lobes bifid, only one auxiliary lobe present. 



This group is commonly supposed to have been derived from the Amaltheidae, 

 but proofs of the connection are lacking. 



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

 Gross. ; Murtoniceras Meek ; Peroniceras Gross. ; Prionotropis Meek. Cretaceous. 



Range and Distribution of the Ammonoidea. 



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

 of the latter about 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 eia. 



Although Ammonites are unknown later than the Cretaceous period, nevertheless 

 this group must be regarded on the whole as the younger brancli 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 

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

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

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



Until a few yeais ago it was believed that only Goniatites and Clj'menias 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 Coal Measures 

 of Texas and in the Lower Carboniferous of the Mississij)pi Valley, pushed their range 



* Danville, //., Involution et classificatiou des Pulcliellides, Bull. Soc. Gcol. France, 1911, vol. xi. 



