ORDER II AMMONOIDEA 573 
Incerta sedis. Lctocentrites, Wilner. Venter crossed by costae that become 
coneave or flattened, and bordered by two rows of tubercles in the young; in later 
stages the venter is rounded and crossed by costae without tubercles. Sutures similar 
to those of this sub-order. Antisiphonal lobe straight, bifid; the first pair of dorsal 
saddles are so large that the extremities are external to lines of involution. 
Sub-Order I. PACHYCAMPYLI. Hyatt. 
| Shells attaining the acme of complexity in evolution of costae ornamented with spines, 
and often combined in the same forms with keeled and channeled venters, Sutures, how- 
ever, are not correspondingly developed, and as regards both number and outlines of their 
lobes and saddles are less complex than in Phyllocampyli. Lobes and saddles as a rule 
shorter, broader, and less arborescent than in either Leptocampyli or Phyllocampyli. 
Characteristic type of sutural outline “ Arietean” or unequal. First pair of lateral 
saddles generally broad, solid-bodied, and bifid ; first pair of lobes adjoining these usually 
much longer and larger than any others. Second pair of lobes and saddles do not 
develop in similar proportions to the first pair, and consequently often appear to belong to 
the later developed auxiliaries. Ventral lobe usually deep, and has a trifid or bifid 
stphonal saddle, which is usually digitated throughout, and never has the elongated, 
triangular, and often smooth outline of that in preceding sub-orders, 
The ventral lobe becomes stouter and broader in proportion, and simpler in outline asa 
rule in normal Cretaceous forms, and these generally have less complex sutures than Jurassic 
shells. Finally, in the phylogerontic Z%ssotidae, Placenticeratidae, etc., the ventral lobe 
becomes much broader and shorter, and the siphonal saddle is proportionally affected. The 
dorsal series of inflections are almost as limited in the primitive Lower Liassic forms as in 
Leptocampyli, consisting only of one very long antisiphonal lobe, a pair of long first lateral 
saddles, and a pair of zygous lobes often Incomplete on the outer sides. Where the dorsum is 
broader and more involute, however, there may be two, three, or more pairs of zygous dorsal 
saddles and an equal number of lobes. These are almost invariably complex in outline, and 
the antisiphonal may be bifid or trifid. 
Phylogerontic forms occur sporadically in the Jura and generally throughout the Creta- 
ceous. They are of two kinds: (1) Those that are retrogressive as compared with their 
ancestors in complexity of sutures only. Sometimes, as in Spheniscidae and Engonoceratidae, 
they are highly involute shells; or, as in Tissotia, etc., they may be more discoidal and 
nodose. (2) Those that are retrogressive as regards‘both form and sutures, like Spirocerutidae, 
Crioceratidae, etc. In these the form becomes uncoiled, following out the tendencies indicated 
in the gerontic stage of progressive forms. Their sutures are reduced to the primitive formula 
of six, i.e. two pairs of zygous lateral saddles and two pairs of lobes, with a ventral and an 
antisiphonal ; but the outlines generally remain complex even in extremely modified forms 
with, however, some notable exceptions like Baculina acuarius, Quenst. sp. 
I. PSILOCERATIDA. 
Includes only one family, comprising radicals of the Jurassic Ammonoids of this 
sub-order. 
Family 1. Psiloceratidae. Smooth or sometimes plicated shells with keelless 
venters, Lateral and dorsal inflections inclined apicad as a rule, but vary considerably. 
Size of saddles reduced along lines of involution, but there are generally three pairs of 
dorsal saddles. Ventral lobe shorter than in Leptocampyli, and siphonal saddles larger, 
as in Arietidae. Aperture with a short rostrum and lateral sinuses resembling those of 
Arietidae, but rostra less acute than in keeled forms. 
Psiloceras has the form and shell characters of Monophyllitidae, and in subsequent stages 

1 The Lenticeratidae and Placenticeratidae furnish the principal exceptions, since they have the 
primitive first lateral saddles beginning to split at an early stage into what afterwards develop into 
three pairs of practically independent lobes and saddles, whereas the primitive first lateral saddles 
usually persist as bifid or trifid saddles throughout life in other forms of this sub-order. 
