216 INFERIOR OOLITE AMMONITES. 



Except for the points mentioned above the figures of this species by the various 

 authors cited at the heading are certainly unmistakable. The description by Mene- 

 ghini is very concise and interesting ; and he also refers to a description by Giebel 

 (' Fauna der Vorwelt,' iii. p. 523, 1852) which I have not had the opportunity to 

 study. Besides this he gives as a synonym Ammonites depressus, Zieten, ' Verst. 

 Wiirtt.,' p. 7, pi. V, fig. 5 ; but as Zieten's work is not at present at hand I cannot 

 venture an opinion on this point. 



Poli/pledns discoides is an unmistakable species on account of its very sharp 

 ventral area, its very small umbilicus, and its complicated sutures. It seemed to 

 be an entirely isolated form ; and for a long time its descent, and consequently its 

 correct place in a system of classification, remained a complete mystery to me. 

 At one time, deceived entirely by its general shape, and by its lobe-line, especially 

 the peculiar accessory tuft on the siphonal lobe (PI. XXXVII, figs. 1, 4, 5), and 

 misled by d'Orbigny's figure of the suture, I expressed an opinion that it was 

 " one of the earliest forms of Oj^^^eli a -like species ; "^ and I also referred it to the 

 genus Oxynoticeras, although with a query, thinking it possibly a highly-developed 

 form of some members of that genus. Now, however, its descent is clearly 

 proved by the small specimens from Milhau, which show exactly how the suture- 

 line developed as the specimen grew ; and consequently I have much pleasure in 

 confirming Hang's view that this species is closely connected with Harpoceras siih- 

 phmatum. The fact of the matter is that this species is a highly-developed form 

 of Harpoceras, is directly descended from Harpoceras suhplanatmn, and is conse- 

 quently the highest developed form of an ascending series — a form in which the 

 ventral area has attained the greatest degree of sharpness, the umbilicus has 

 become nearly closed up, and the sutures have been made most complicated in 

 order to support the broad sides of the whorl.^ 



I am of opinion that it is advantageous to give this species a distinct generic 

 name ; because it differs in so many important respects from Harpoceras, and it 

 develops its peculiar characters at a very early age. The sharpened ventral area 

 without, so far as I can see, any trace of the hollow-carina of Harpoceras — 



^ This Monograph, p. 40, foot-note. 



^ It is worth while to notice another rather tempting and suggestive theory, namely, that Pol- 

 discoides is the further involute development of Ammonites kurrianus (Oppel). Meneghini has 

 figured a species* which is in position and the amount of involution exactly iutermediate between 

 kurrianus and discoides ; but, unfortunately, he gives no suture-line. I believe, however, that the 

 suture-line observable on young Polyplectus discoides really disposes of the matter, and that Harpo- 

 ceras is the true parent, although in general appearance, and especially in the shape o£ the ventral 

 area, the species above mentioned are very similar. 



* Ammonites, sp. ind., Meneghini, ' Fobs. calc. rouge, Pal. Lombarde,' 4e Serie, pi. ix, fig. 1 ; 

 Harpoceras, cf. Kurrianum, ibid., p. 199. 



