POLYPLECTUS DISCOIDES. 215 



PoLYPLECTUS DISCOIDES {Zieten). Plate XXXVII, figs. 1 — 5. 



1830. Ammonites discoides, Zieten. Verstein. Wiirtt., pi. xvi, figs. 1 a, b, c. 



1844. — — d'Orhigny. Terr, jurass. ; Pal. Fran9., pi. cxv. 



1846. — CAPELLiNus JUEENSis, Queustedt. Ceph., p. 106. 



1856. — DISCOIDES, Oppel. Juraf., p. 245. 



1858. — — Quenstedt. Der Jura, pi. xl, fig. 7, p. 283. 



1867. — — Meneghini. Foss. calc. rouge ; Pal. Lombarde, 



4e Scrie, p. 20. 

 1867. Leioceeas discoides, Hyatt. Cepb. ; Bulletin Mus. Comp. Zool., p. 102. 

 1874. Ammonites discoides, Dumorlier. Bassin du Ebone, iv, p. 54. 

 1881. Haepoceuas discoides, Meneghini. Eevisionsystematique; Pal. Lombarde, 



4e Serie, p. 199. 



1884. — — Wright. Monogr. Lias Ammonites ; Pal. ISoc., 



vol. xxxviii, pi. Ixxxii, figs. 12, 12 a, 13. 



1885. — — Haug. Beitr. Monogr. Harpoceras ; Neues Jabr- 



bucb fiir Mineralogie, &c., Beil.-Bd. iii, 

 p. 620. 



The description of this species is given under the generic definition. 



Side and front views of this species have been given by Dr. Wright, who stated 

 that, as none of his specimens showed the lines of the structure of the suture, the 

 suture-line (fig. 12 a) drawn by his artist was a copy of that given by d'Orbigny 

 (op. cit., pi cxv, fig. 4).^ 



The suture-line of this species increased in complexity as the shell grew, so 

 that it becomes extremely difficult to follow it on adult examples. The 

 clearest part of the suture-line is towards the outer edge ; and this is just where 

 d'Orbigny's mistake is, namely, that the accessory lobe in the siphonal saddle is 

 nothing like large enough. This is a serious mistake. There are some other 

 errors in the suture-line, as I will presently point out; but they are of minor 

 importance. The same mistake appears in d'Orbigny's front view — there 

 is no accessory lobe depicted in the siphonal saddle. It is in order to 

 rectify this mistake that I have had the present figures drawn (PL XXXVII, 

 figs. 1 — 5). The suture-line I have traced from a specimen obtained at White 

 Lackington, which shows the sutures in many places with great clearness ; 

 and the details of these sutures are confirmed by two little specimens from Milhau. 

 Aveyron (France), which are also entirely without test. One of these specimens 

 furnishes the front view (fig. 3). 



^ ' Monograph on the Lias Ammonites,' p. 467. 



