300 :siE. s. s. BrcKMA?^" ois" [vol. Ixxiii^ 



If these forms were to be combined under one name as Arnioeeras 

 miserahiJe without taking account of the differences of periphery 

 and proportions, it is possible that certain facts of geographical 

 and stratigraphical distribution might be overlooked for want of 

 distinctive appellations. It is not yet known that the different 

 forms are of strictly identical dates ; and on the geographical 

 distribution something will be said later (p. 314). 



Facts of evolution might also be lost without distinctive names. 

 For there seem to be realty two lines developing side b}^ side — one 

 in which the periphery proceeds no further than the angulate stage 

 but the whorl increases laterally — in thickness ; the other in which 

 the periphery i^roceeds past the angulate stage to a carina with 

 indications of channels, but the lateral development lags behind — 

 the increase in whorl-thickness is slower. Of the first series 1-3 

 are morphic equivalents of 1, 2 of the second ; -1 of the first is in 

 the same relation to 3 of the second, while 4 of the second stands 

 isolated. 



To Hyatt's idea that the smooth forms of Arnioeeras are 

 derivates of T'siloceras ])lcnior})is I cannot assent. Ps. i^lanorhis 

 and like smooth forms of that genus are catagenetic — post-costate 

 — developments wTbh a degenerating phylloid suture-line. The 

 smooth forms of Arnioeeras are anagenetic developments — pre- 

 costate — with a ceratitic suture-line, simple because undeveloped. 

 The likeness is deceptive : it is a case of transversal homoeomorphy.^ 



The next species well illustrates the precostate character of 

 smooth Arnioeeras. 



History of figured specimen. — From the Lias of L3^me 

 Regis (Dorset) — purchased there. It is yellowish-brown — hence 

 the trivial name — the colour which Sowerby illustrates for his 

 Ammonites hroolci!^ This character may perhaps be a means of 

 fixing its exact stratigraphical position. See later, p. 314. 



Ae:n^ioceeas ain-a&exeticum, sp. nov. (PI. XXXI, figs. 3 «-3 i.) 



Cf. 1889. Arnioeeras semicostaUim Hyatt {non Young L Bird sp.), 'Gen. Ariet.'' 

 pi. ii, tig. lU only.— Proportions : F. 34, 26, 26, 5]. 



Proportions. J S. 24, 33, 25, 44. 



I S. 42, 27, 20(24), ol. 



Description. — Serpenticone ; subplat^^-subleptogyral ; subex- 

 tremilatumbilicate ; carinati-subsulcate; mainly smooth, finishing 

 costate, parvinodate. 



General details. — The distinctive feature of this species is a 

 prolonged smooth or nearly smooth stage, for over most of this 

 stage capillation and occasional very obscure costulae may be 

 detected. On the last quarter-whorl are developed what must 

 be regarded as adult or even old-age characters — costse plain at 

 first, but later on bearing incipient tuberculation — small nodes. 



^ ' Yorkshire Type Ammonites ' ii (1913) p. vi. 

 '^ ' Mineral Conchology ' 1818, pi. cxc. 



