YORKSHIRE TYPE AMMONITES Dec. 



Most — perhaps all — of the Lias Planulati (Dadylioceras, etc.) have 

 each rib parted off by a septum : they are septicostate ; and so the 

 degree of costation as between test present or absent differs considerably. 

 The septicostation is a good character by which to distinguish them 

 from homoeomorphic Bathonian, or other Perisphinctes, Peltoceras, etc. 



The davus and auriculoid. — On the outer lateral area of certain 

 Oxynoticerates there are c;-shaped processes which Pompeckj calls 

 noeuds paraholiques. In Am. auriiulus'- there are knobs longitudinally 

 extended, connecting two ribs in a button and loop style : each of these 

 Quenstedt terms eine ohrenartige schleife (an ear-like loop or knob). 

 A similar ornamentation of stronger, longitudinally elongated knobs is 

 shown in Am. sinemuriensis, d'Orbigny,'^ Am- forficatus, Strickland- 

 Buckman,^ and other Coronicerates. 



These Coronicerates, A. auritulus and the Oxynoticerates present, 

 in three stages of catagenetic development, an ornament which may be 

 called a davus when a strong boss, and an auriculoid when a relic, as 

 in the Oxynoticerates. The importance in the latter is its indication 

 that they are catagenetic developments which have had an ancestor 

 in the tuberculate stage, and hence the ontogeny of Oxynoticerates 

 gives examples of saltative palingenesis. 



If the auriculoids are rightly explained as degenerate longitudinally- 

 elongated knobs (clavi) of the A. sinemuriensis pattern, then their resem- 

 blance to the ornaments which have been called parabolical curves, 

 Parahelknoten, or tubercules auriculaires among Perisphinctes is only 

 accidental. 



A note in regard to Simpson's terms may be necessary. By 

 depressed he means what would now be called compressed ; by radius 

 he intends simply the costa, not its pre- or post-developments — stria ; 

 by back or dorsal he denotes what are now called venter, ventral. As to 

 his measurements, they are certainly only just approximate — in fact, 

 they are rough. 



Ammonite Development 



The need for some of the foregoing terms and for others may be 

 understood from the following remarks on development. 



In regard to the coiling — the amount of evolution or involution — 

 of the conch of Ammonites, there is apparent a course of cychcal develop- 

 ment ; but it is difficult to speak of anagensis or catagensis, of elaboration 

 or simplification, of progression or regression, in regard to the mere 

 changes in the coiling of the conch, considered altogether apart from 

 its ornamentation.* 



I See A. dennyi, No. 7. 2 1844, xcv, i, 2. 3 Geol. Chelt, 1844, 104 ; figd- 

 Proc. Cotteswold F. C. xv, igo6, xi, 8, 9. 



4 Hyatt regarded increase of involution as progressive; Gen. Arietidae, 71. 



