OILDBH ii A.MMoNolDKA 



Permian ami Tria^.-ic genera h>-iv a., iated in lln- .-aim- ,-ub-ordei -an- parallel with 

 normal Triussir ( Vrat itoids, ami have a wide di.-t ribut ion in tin- Tria-. The ' 

 <////>//// is an aberrant stock \\ith pe.u liar sutures. They be^-in in tin- Devonian, Inn 

 have not, yet been traced with certainty in tin- ( 'arboniferoii-. The\ re.ippear, liow 1 - 

 ever, in tin- Permian, ami arc represented by a Dingle family in tin- Tria- of India and 

 Northern Siberia. Tin- Devonian ami Permian genera arc widely di-t ributed. 



(3) Ceratitoula. --The sub-order l>is,;,,;iinjiiili i- re-tridcd to tin- Tria- \\ith the 

 exception of a single family, /'/ /////* iiiti<ln<; which occur.- in tin- Permian of India 

 and North America. The form- have a world-wide di-t ril.ut ion, and are e.-pecially 

 numeroiis in India and the western l/nited State.-. Tin- group attain- the acme of 

 specialisation among Ammonoids, as shown ly their extremely complex -ntnie- \\ith 

 numerous lobe< and .-addle-, comliincd with a highly ornamented exterior and con- 

 siderable modilicatioii of form. The numlier of ret rogressive or phylogerontic 



i- also very small. 



(4) Ammonitoids. The Phyllocampyli include the supposed ancestral and piimiti\e 

 forms of typical Ammonoids, and begin, with the family Protoani&fcu, \\hich 



from the Devonian to the Permian. Three primitive offshoots of this family aie 

 present ill the Permian, two of which persist al.-o during the Trias. In all, nine 

 families of Pkyttocampyli occur in the Trias, only one of which (PhyUoceratida* 

 continues throughout the Jura and Cretaceous. Most of the families have a world- 

 wide distribution in the several formations where they occur. The sub-order rea<he- a 

 high degree of sutural complication, and this, together with the number of lobes and 

 saddles in the Phylloceratidae, mark them as being among the most progressive <>t 

 Ammonoids in respect to sutures. Their shells, however, are very simple in comparison 

 with either the Discocampyli or Pachycampyli. 



The Leptocampyli are obviously a retrogressive series. They bc^in at the i 

 the Jura with forms having a restricted number of lobes and saddles, and terminate in 

 the Upper Cretaceous with two families of uncoiled phylogerontic .-hells. The com- 

 plexity of the sutural outlines is, however, constantly maintained, as i> especially well 

 illustrated by Lytoceratidae, where the arms of the antisiphonal are curinu-ly pro- 

 longed. Their distribution coincides with that of the riiyllm-i'infiilf, and tl, 

 quite abundant in certain Jurassic and Cretaceous localities 



The Pachycampyli are usually considered as the typical and al>u mo.-t < 

 forms of Ammonoids. This is true as regards the immense number and variety of 

 modifications in form and ornament displayed by their shells and apertures ; but the 

 sutural inflections are on the average less complex both in outline and number of 

 lobes and saddles than those of the Phyllocampyli, although both group.- are evident 1\ 

 derived from the same stock. 



By the term progression, as used above, is meant ' evolution of structures increasing 

 by differentiated additions," such as the addition of more complex inflection!* to either 

 the dorsal or external sutures, increase in the amount of involution, or the introdm - 

 tion of keels and channels on a primitively convex venter, etc. Retrogression signifies 

 "evolution through the reduction of such progressive characters," whether taking 

 place locally or generally in the organi-m or throughout the group. Retrogression, in 

 this structural sense, does not imply actual reversion to ancestral conditions, except in 

 so far as the disappearance of a part or organ necessarily produces a certain r -emblam e 

 to their ancestors before the parts or organs in question were evolved. Kxamjde.- will 

 be found on comparing Baculites with Bactrites, Mimoceras with Criocera*, etc. In 

 this sense Ammonoids experienced a progressive evolution from the early Devonian 

 until the Upper Trias, when the first signs of general retrogression are ob-ened, and 

 a few uncoiled and turriliticonic genera appear. 



Following their culmination in the Trias, Ammonoids display in the Jura a 

 mixture of retrogressive with some progressive features. Part of their losses are 

 regained by the evolution of a vast number of forms and modifications during this 



