ORDER II AMMONOIDEA 553 
represented in primitive species, and are more or less traceable even in specialised shells. 
Aperture with hyponomic sinus. Shells smooth or with longitudinal grooves. 
Examples: Dimorphoceras, Nomismoceras, Hyatt. Carboniferous. 
Family 5. Thalassoceratidae. Discoidal and involute ammoniticones with 
undivided ventral lobes in primitive forms, and also entire lobes and saddles; but in 
more specialised shells the former are divided, and the latter digitated by marginals, 
but bases of saddles may remain entire. Outlines of sutures in primitive forms indicate 
affinity with Nomismoceras, and probable derivation from same stock. Aperture with 
hyponomic sinus. Shells smooth. 
Examples: Paraceltites, Thalassoceras, Gemm. Permian. 
II. NANNITIDA. 
A provisional group, including smooth, more or less discoidal and globose keelless forms 
with primitive sutures. The latter pass through a Magnosellarian stage in the young, but 
subsequently assume the aspect of Dimorphoceratidae. Ventral lobes retain entire siphonal 
saddles throughout life. Specialised forms have elevated subacute and often keeled venters. 
Family 6. Nannitidae. Anarcestean forms with a tendency to become elevated 
on the venter in later stages of growth, and having sutures with faintly denticulated 
lobes and entire saddles, and with very small entire siphonal saddles. 
Nannites, Prosphingites, Mojs. (?) Sphaerites, Arthaber. Trias. 
Family 7. Otoceratidae. Involute forms with deep umbilici and prominent 
ridged umbilical shoulders; venter keeled and acute. Ventral lobes divided by large 
siphonal saddles. Young have volutions with broad keelless venter and trapezoidal 
section. Antisiphonal lobe, so far as known, entire on the sides and bifid at the ends. 
Otoceras, Griesbach ; Anotoceras, gen. noy. Type A. (Prosph.) nala, Diener sp. 
Trias. 
III. TrRoprripa. 
Includes a large and comparatively homogeneous group of Triassic shells of Anarcestes- 
like aspect, but sutures with peculiarly elongated and completely digitated saddles and lobes. 
Siphonal saddles not so deeply incised by marginal lobes as the laterals, nor very complex 
in outline. Shells ornamented with costae, often tuberculated, keeled, and channeled. 
Aperture invariably with lateral crests. Connects through Microtropites with Nannites. 
Family 8. Tropitidae. Mojsisovics (pars). Similar to Anarcestidae in the 
ephebie stage, but the young frequently have volutions with highly trapezoidal 

Fic. 1135. 
Marqarites Jokelyi, Hauer sp. Fig. 1136. l 
Upper Trias; Sandling, pear Tropites subbullatus, Hauer sp. Upper Trias ; 
Aussee, Austria. Aussee, Austria. 1/;. 
sections. Shells highly ornamented by the intersection of a system of longitudinal 
ridges and transverse costae, Sutures have deep, narrow ventral lobes divided by 
siphonal saddles with peculiar truncated bases, which are often retained in later stages. 
Sutures in ephebic stages of some forms are similar to those of Haloritidae; young 
have a more or less prolonged coronate stage, and are also keeled on the venter as a 
rule. Aperture narrower and with more pointed ventral crests than in Haloritidae. 
