590 



MOLLUSCA 



SUB-KINGDOM VI 



Family 37. Hystatoceratidae. Similar in general aspect to Peroniceras. 

 Young compressed, smooth, and keeled until a late stage; they then become costated, 

 venter loses its keel and is crossed by fold-like costae. Sutures simpler than in 

 preceding families and resemble Tissotia except that the first lateral saddles are 

 symmetrically bifid. Dorsal sutures have at least two pairs of zygous saddles and one 

 of zygous lobes ; sometimes three pairs of saddles and two of lobe.-;. 



Hystatoceras, gen. nov. (Brancoceras, Steinm. non Hyatt). Type H. (Amm.) 

 Senequieri, d'Orb. sp. ; Hysteroceras, gen. nov. Type H. (Amm.) varicosus, d'Orb. sp. ; 

 Falloticeras, Par. and Bon. Cretaceous. 



Family 38. Lenticeratidae. Young smooth, keeled, discoidal, and resemble 

 preceding family, but adult sutures like those of Placenticeras. Saddles very broad 



and comparatively solid, the first laterals 

 usually with three arms. Arietean lobes not- 

 present. Adventitious saddles arise by division 

 and outgrowth of the ventral or outer sides .of 

 the first laterals as in Tissotidae. 



Lenticeras, Gerhard ; Paralenticeras, geii, 

 nov. Type P. (Amal.) Sieversi, Gerhard 

 sp. ; Platylenticeras, gen. nov. Type P. 

 (Oxynot.) heteropleurum, Neum. and Uhlig sp. 

 Cretaceous. 



Family 39. Tissotidae. Primitive forms 

 have tuberculated costae and keeled venters, 

 the keel probably having originated by coal- 

 escence of a median line of tubercles. Sutures 

 usually very simple, sometimes similar to 

 P idchellidae, and first lateral saddles apt to be 

 unequally bifid. Adventitious saddles appear 

 in some forms, but arise by division of t In- 

 cuter sides of the first laterals instead of from 

 their inner sides as in. Placenticeratidae. 



Tissotia, Douville (Fig. 1235) ; Psilotissotia, 

 gen. nov. Type P. (Pulch.) Chalmasi, Nickles 

 sp. ; Lopholobites, gen. nov. Type L. (Neol.) 

 Cotteauxi, Nickles sp. [? Hemitissotia, Plesiotissotia, Heterotissotia, Peron. Names only 

 reported in Revue Grit. Pal. 1899.] Cretaceous. 



Incertae sedis : Buchiceras, Hyatt ; Mojsisovicsia, Steinmann. Cretaceous. 



Tissotia Fourneli, Bi 

 el-M'.sai, Algiers (after 



FIG. 1235. 



le. Cenomanian ; Mzab- 

 ayle). 



Range and Distribution of the Ammonoidea. 



Not less than 5000 species of Ammonoids have been described, as contrasted with 

 about 2500 of Nautiloidea. The distribution of these amongst the different sub-orders 

 may be briefly indicated as follows : (1) The Gastrocampyli are an aberrant stock con- 

 fined to narrow limits of the Upper Devonian, and restricted geographically to a few 

 localities in Germany, Russia, and England, with only a single species from the 

 United States. 



(2) Goniatitoids. The Microcampyli comprise primitive ancestral forms of Ammo- 

 noids, and are confined to the Devonian system. The Mesocampyli are an aberrant 

 division occurring in both the Devonian and Carboniferous systems, and for the most 

 part enjoy a wide distribution. The most highly specialised family Pinnacitidae is 

 limited, however, to only a few European localities. The Eurycampyli is by far the 

 most important group of Palaeozoic Ammonoids, and is represented throughout the 

 Devonian and Carboniferous of both hemispheres by typical Goniatitoids. The 



