POLYPLECTUS DISCOIDES. 217 



the absence of a definite ventral area — the almost occluded whorls — the ribs 

 without the sharp falcate shape — the complexity of the suture-line generally, and 

 the manner in which the chambers are crowded one into another — the large 

 accessory tuft in the siphonal saddle — the large number of auxiliary lobes — all these 

 are points which separate this species entirely from HarpoceraH, sennu stricto ; and 

 although they maybe regarded, when taken together, as the consummation to which 

 the series of Ilarpoceras falcifermii was making more or less progress, yet it is 

 just because this consummation has been obtained so completely in Pol. discoides 

 that the species is so distinct from its forerunners in so many respects, and that 

 it seems to be entitled to have its peculiarities duly noted by a distinct generic 

 or subgeneric appellation. 



The development of the suture-line of this species fully bears out my remarks 

 at p. 135, that the suture-line increased in complexity after the involute shape had 

 become a fixed character. The ascending series having attained, as Ilarpoceras 

 suhplanatum, nearly as broad a side as it was possible to acquire, had yet not altered 

 the suture-line to any great degree ; but the broad whorls having become a definitely 

 fixed character, we find that Polyplectus discoides begins, even when quite small, to 

 increase the complexity of its suture-line, presumably to give adequate support to 

 its broad, flat whorls. The development of the accessory tuft is also entirely in 

 accordance with the loss of the carina and ventral area, and the acquirement of a 

 very sharp ventral edge ; because this change has converted the branches of the 

 siphonal lobes into lobes supporting the outer edge of the lateral area. These 

 branches have, in Pol. discoides, to support a flattened portion of the lateral area, 

 instead of, as in Harp, subjjlanatum, an arched ventral area. A complicated 

 suture-line with an accessory tuft to the siphonal saddle is therefore no argument 

 for the relationship of this form to Oppelia. It is true that the accessory tuft is 

 also found in most species of that genus ; but its production is a necessary result 

 of a similar change in the relative shape of the ventral and lateral areas. A 

 similar augmentation of the branches of the siphonal lobe — but without a definite 

 accessory tuft — is to be seen in the sutures of Amaltheus margaritatus ;^ and I 

 have stated that this is due to the same cause.^ 



Not only is the suture-line of Pol. discoides more complicated in general on 

 account of the extra growth and greater division of all the small follicles of the 

 lobes, but an extraordinary array of auxiliary lobes is put forth — a great advance on 

 what obtains in Harp, suhplanatum. D'Orbigny gives four not very clearly defined 

 auxiliary lobes for Harp, suhplanatum {Am. complanatus, pi. cxiv, fig. 4) ; while he 

 gives six distinct auxiliary lobes for Pol. discoides (pi. cxv, fig. 4). This is, of 



^ " The Descent of Sonninia aud Hammatoceras,'' ' Quart. Jouru. Gaol. Soc.,' vol. xlv, pi. xxii, 

 fig. 5. 



- Ibid., p. (j5G. 



28 



