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 Tvssotia 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 lobes. 
Hystatoceras, gen, nov. (Brancoceras, Stemmm. non Hyatt). Type H. (Ami.) 
Senequiert, VOrb. sp.; Hysteroceras, gen. noy. Type H. (Amim.) varicosus, VOrb. 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 Tvssotidae. 
Lenticeras, Gerhard; Paralenticeras, gen. 
noy. Type P. (Amal.) Sieverst, Gerhard 
sp.; Platylenticeras, gen. nov. Type P. 
(Oxynot.) heteropleurwm, 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 
Pulchellidae, and first lateral saddles apt to be 
unequally bifid. Adventitious saddles appear 
in some forms, but arise by division of the 
outer sides of the first laterals instead of from 
their inner sides as in Placenticeratidae. 

Fic. 1235. 7 5 5 z < . 5 5 
Fae AG; 7 ss oe ae Tissotia, Douville (Fig. 1235) ; Psilotissotia, 
issotia Fourneli, Bayle. Cenomanian ; Mzab- : / . * 
el-M’sai, Algiers (after Bayle). gen. nov. Type P. (Pulch.) Chalmasi, Nickles 
sp.; Lopholobites, gen. noy. Type L. (Neol.) 
Cotteauxi, Nickles sp. [? Hemitissotia, Plesiotissotia, Heterotissotia, Peron. Names only 
reported in Revue Crit. Pal. 1899.] Cretaceous. 
Incertae sedis: Buchiceras, Hyatt ; Mojsisovicsia, Steinmann. Cretaceous. 
Range and Distribution of the Ammonoidea. 
Not less than 5000 species of Ammonoids have been described, as contrasted with 
about 2500 of Nauwtiloidea. 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 systém. 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 Euryeampyli 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 

