40 I'Kor. U. II. SWIXXEKTON AXi) Ml{. A. K. TlfrEMAX Tvol. Ixxiil, 



characturize tin* first stages in the simplitieation of tlie suture-line, 

 a simplitieation that is earried to sueh extremes in Cretaeeous 

 ammonites. 



IX. The Adult Sltike ue UAcrvLiocEUAs commlwe. 



The adult suture (tig. 1) /, Hg. U) m) shows externally a deep 

 ventral lohe divided by a median saddle, and Hanked by broad external 

 saddles. The first lateral lobes are almost as wide, but not so deep, 

 as the ventral lobes ; while the first lateral saddles are of smaller 

 l)i'opoi'tions than the external saddles, presenting in this respeet a 

 marked eontrast with Tra(/ophi/Uoceras. The second lateral lobes 

 are very small, and the two auxiliary saddles of the external series 

 are insignificant. Internally, the dorsal lobe with its minute 

 median saddle is the most important feature, being exceeded in 

 depth only by the corresponding lobe of the external series. A 

 pair of internal saddles, about as high Ijut slightly narrower than 

 the first lateral saddles, and a pair of smaller auxiliaries separated 

 from the internal saddles by internal lobes, are also present. All 

 the features of the suture-line, except the auxiliaries, are compli- 

 cated by the presence of frillings or denticulations. On each of 

 the external saddles are three main divisions similar to the simple 

 divisions on the internal saddles. The first lateral lobe of the left ^ 

 is markedly trifid. That of the i-ight side is abnormal. A similar 

 asymmetry has been noticed in other ammonites, as, for example, 

 in Sonninia.^ While these major frillings are symmetrically 

 developed on each side of the median line, the minor frillings are 

 less constant, and frequently give rise to asymmetry. Thus, 

 although the three minor fiillings on the central division of the 

 external saddle are present on each side, the small frills on the dorsal 

 foliole of the same saddle are much more developed on the left 

 than on the right side. It ap})ears. therefore, that, at least in 

 such retrogressive types as DactyUuceras, only the major frillings 

 can be of systematic importance. The minor frillings, which 

 further complicate the suture, show even on opposite sides of the 

 same suture an inconstancy that may be associated with the 

 })henomena of decline discussed above. 



X. Deyelopmext of the Suture tx DAcryrjocEUAs. 



The first suture (fig. 'da, p. 42) of Dacti/Uoceras shows the 

 usual angustisellate condition. 



In the second suture (9 i ) a well-defined ventral lobe is ])resent. 

 which is as broad as it is deep. The dorsal lobe is similar, but 

 smaller ; both are deeper than the lateral lobes. As in the adult 

 suture, the external saddles are the largest, and the internal saddles 

 come next in size. The lirst lateral saddle is just appearing as a 

 low fold close to the umbilical angle. 



' Ki",'!)! :ui(l k'l't ;is uslmI in tla- (If^c-riiition itRt oiil}- to tlio figures. 



- S. S. liuL'kinan, ' liif, Oul. Ammuiiites ' Muiiugr. Palttunt. Sue. ISUi. p. 381. 



