50 PROF. H. H. SWI>'NERTON AXD MR. A. E. TRUEMAX [vol. Ixxiii, 



sucli as the auxiliaries of suture-lines, the last-formed plates in the 

 stalk of a crinoid, or in the interambulacral area of an echinoid. 

 The simplicity of the central area of the adult septum is due to 

 the retention of this feature through all stages of development, 

 despite the increasing complexity of adjoining parts. 



Just as the development summarizes the stages of evolution 

 with omissions here and additions there, so septal sections summarize 

 the development, but with omissions ou]j. The features to be 

 omitted are determined by the fact that the series of changes, 

 shown in the passage from the centre of the septum to the edge, 

 represents the most direct way in which the adult septum, with 

 its highly complicated margin, could have been evolved from a 

 septum as simple as that of an asellate protoconch. If this course 

 of evolution has been, as it were, a winding one, the winds Avill be 

 omitted, to some extent in development, and completely in the 

 adult septal sections. 



In ammonites with normally-shaped Avhorls the septal sections 

 closely resemble the sutural development : in highly-compressed 

 and depressed forms, however, the central area of the septum is 

 like a primitive septum greatly elevated or laterally extended. 

 During ontogeny, the first lateral saddles in these forms early 

 assume adult proportions, and consequently in septal sections the 

 stages represented by the earliest sutures are omitted. In no case 

 do septal sections show a stage comparable with the fii'st suture. 

 This seems to indicate that, in ammonites, a change akin to a 

 metamorphosis took place in the period between the formation of 

 the first and the second septum. The creature which secreted the 

 former was possibly larval, and followed a line of evolution of its 

 own : which is reflected in the series of coenogenetic changes repre- 

 sented by the terms asellate, latisellate, and angustisellate. 

 Further, in such forms as Pinacoceras, Avhere a large number 

 of adventitious inflections are developed, and" compression has 

 reached its maximum, we should not expect to find so close an 

 agreement between the septal sections and the sutural development, 



With these exceptions, however, septal sections will at least 

 furnish a standard of direct development, ^ which may be useful foi* 

 detecting deviations in the normal development where such occur. 

 This direct development may or may not approach as closely as 

 ontogeny to a true reprjsentation of actual evolution. 



Septal sections should prove especially useful where material for 

 the study of early stages is not accessible. Although, in ordinary 

 cases, sutural development may prove of greater value than the 

 septal sections, where the former cannot be worked out the latter 

 will, to some extent, serve as a substitute. 



It is well known that the sutures of senile stocks bear a close 

 resemblance to the young sutures of the advancing types which 

 preceded them. This resemblance is due, not to a resumption of 



' We owe this sug-g-estion to a conversation with tliat brilliant and muoh 

 lamented investigator, the late Dr. A. Vanghan. 



