part 1] THE MOBPHOLOGT OF THE AMMONITE SEPTUM. 47 



As in Dactjflioceras, the smaller details of the suture are so 

 asymmetrical about the median line as to indicate that they are of 

 little consequence in taxonomic discussion. Here, again, the com- 

 parison of the developmental series and the septal sections may he 

 confined to the more outstanding features. The sections give 

 nothing compara])le with tlie first suture, but the innermost 

 sections (hg. 13 h & c) are almost identical with the seventh suture 

 (fig. 12 c). This similarity is emphasized by the presence of tlie 

 ventral saddle which, by acceleration in development, appears earlier 

 in development in Tragopliylloceras than in Dacti/lioceras : a 

 difference in the time of appearance which is manifested also by 

 the sections of the two t3^pes. As in Dactylioceras, the fresh 

 lobes and saddles appear at the umbilical angle in both sutures and 

 sections. In the sutures, the first denticnlation is shown on the 

 external saddle, new^ frillings coming in rapidly from that time 

 onwards. Although the relative proportions of these divisions are 

 not the same in the sections as in the sutures, the numbers at the 

 various stages are practically identical. The difference in pro- 

 portions may be partly explained b}^ the difference between 

 Mr. Spath's specimen and ours, as mentioned previousl}^. It may 

 also be suggested that the notches which produce ' undercuts ' on 

 the folds are confined to the edge of the septum, and are therefore 

 necessarily shown only in the sutures, 3''et are absent from the 

 inner portions of the early as well as the latest septa. This last 

 fact is of practical value in breaking up an ammonite for examina- 

 tion : for, if the outer surface is filed away along any suture-line, 

 the chambers may be readily separated. 



XIII. Septal Sections of SpH.imocEiiAS BROxGyiAitTT 

 (J. Sowerby). (See figs. 15 & 10, p. 48.) 



The septal sections of this specimen correspond very closely 

 with the developmental series. The order of appearance of denti- 

 culations in ontogeny is closelj'^ paralleled in the sections. Thus 

 the first frillings in each series are developed in similar positions 

 on the external saddles (compare fig. 16 c & d with fig. 15 r) after 

 the appearance of the median saddle in the ventral lobe. At this 

 stage there are tliree jmirs of saddles in both cases. The develop- 

 ment of additional frillings and of small auxiliaries also proceeds 

 along almost identical lines. It will he noted, however, that in onto- 

 geny the median ventral saddle is present at an earlier stage than in 

 the sections : namely, befoi-e the development of the second lateral, 

 saddle. The differences noted in the previous comparisons are also 

 to be seen here ; the undercuts, which are well developed in onto- 

 gen}^, are only preserved in the outer parts of the septum, and 

 consequenth^ do not appear in any but the outermost sections. 

 Further, even in the inner septal sections, main folds are genei-ally 

 more acute than in development. This is due to the absence of a 

 central flattened area. 



