ORDER i r A.M.MnNnlhKA 578 



Incerta sedis. AV/oo,, ////<>, Wahner. Venter OKMMd 1-y OOttae that become 

 eoneave or flattened, and hordeied l.y t\\. POW8 of tiil.eivl,- in tln-voim^; in later 

 Stages the venter is rounded and CTOed l.y OOtUw without tul.er.-h-. Sutm.- Minilar 

 tO those of this sub-Order. Aiiti>iph<>iial h.U- -trai^ht, liif'nl ; thefir-t |.air.,t 

 saddles are so large that the extremities are exteni;il to lines of involution. 



Sub-Order I. PACHYCAMPYLI. Hyatt. 



Shell* nffiiiiliilil (In- IK-IIII' >if Cn,, ij,l,:rit, ( in ,-i;,lntii>ii i,f i;,stnr ,,,-,>" ////,-// ,//'//, , 



and often combined in the same forms with keeled and channeled / /' rt. 8utw 

 ever, are not correspondingly developed, and as regards both number //// ///////< > >,f ///.// 

 lobes and sa<l</l'* n n- Irs* cnnij>l,.i- tlnm in I'liiillnnmijiiil/. J.iiln-s and saddles as mi- 

 shorter, broader, and less arborescent than in either Leptn,-nii,j>iili /// /'/<////"<" /;,y,,///. 

 Characterittic //// <>/ aiifunif <> nt/in <> " A rietean" or unequal. First /mi/- ,,f /,//,,-,/ 

 saddles generally broad, solid-bodied, and bifid ; first pair of lobes ii</j<,ii,i,,<i t !,,.<. />.<//////,/ 

 much longer and larger than any others. 1 Second pair of lobes ami x/f/i//-x il<> //,/ 

 develop in similar proportions to the first pair, and consequently often appro i' tu b<l<nui t<> 

 the later developed auxiliaries. Ventral lobe usually deep, and has a trifid or bifid 

 siphonal 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 

 rule in normal Cretaceous forms, and these generally have less complex sutiuv> than .Im 



as a 



shells. Finally, in the phylogerontic Tissotidae, 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 form 

 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 dnr>imi is 

 broader and more involute, however, there may be two, three, or more pairs of zygous d>r.-nl 

 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 tipheniscidae and Engonoceratidac, 

 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 Xjnrm'i rut iiln, . 

 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 tnninil.i 

 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 f>nii- 

 with, however, some notable exceptions like Baculina acuarius, Quenst. sp. 



I. PSILOCERATIDA. 



Includes only one family, comprising radicals of the Jurassic Ammonoids of thin 

 sub-order. 



Family 1. Psiloceratidae. Smooth or sometimes plicated shells with keelless 

 venters. Lateral and dorsal inflections inclined ajiieud as a rule, luit vary i mi-iderably. 

 Size of saddles reduced along lines of involution, but there are generally three j.air> 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 Lentice,ratidae and Placenticeratidae furnish the principal exceptions, since they have tin- 

 primitive first lateral saddles beginning to split at an early stage into what afterwards .levi- 

 three pairs of practically independent lobes and saddles, whereas the primitive first lateral 

 usually persist as bifid or trifid saddles throughout life in other forms of this sub-order. 



