214 INFERIOR OOLITE AMMONITES. 



I know nothing of the sutures of this species, but, judging from the shape of 

 the strige, it belongs to the genus Grammoceras. The striae are not at all falcate, 

 as would be the case if it belonged to Harpoceras. 



I can form no conjecture concerning the descent of this species ; its evolute 

 whorls are a most unexpected feature considering its position and the state of its 

 other characters. It must be regarded as a degraded form of the genus, and 

 that it had, as to the umbilicus, reverted to the more ancestral shape. 



Not only is this species distinct in appearance from its congeners, but it is 

 separated from them in time ; for it occurs in the Murchisonse-zone, and is, so far 

 as I know, the solitary representative of the genus on that horizon. 



Only two specimens of this most interesting form are known to me ; and I 

 have not had the fortune to find either myself. One was in my father's collection ; 

 it came from Bradford Abbas, Dorset ; and judging by its matrix I have no hesi- 

 tation in saying that it came from the " Paving-bed." The other specimen was 

 obtained for me from Stoford, Somerset, by Mr. F. Stubbington. PI. XXXIII, 

 figs. 13, 14, exhibit two views of the specimen from Stoford, showing the trenchant 

 carina. Figs. 15, 16, give two views of the specimen from Bradford Abbas. 



HiLDOCERATiDiE (continued). 

 Oenus — PoLYPLECTUS,^ ;S^. Buchman. 



Definition. — Discoidal, compressed, highly-involute. Whorls, in section, 

 acutely-triangular, without ventral edge, but with a very sharp outside; with 

 broad sides, ornamented with sigmoidal radii strongly-projected on the outer area. 

 No separate carina. Sutures extremely complicated — siphonal lobe large, and 

 with an accessory tuft — siphonal saddle with a large accessory lobe — superior 

 lateral lobe shorter than the siphonal — seven or more auxiliary lobes. 



Remarks. — Only one species, namely, Polyplectus discoides (Zieten), is included 

 in this genus. It is a descendant of Harpoceras ; but the changes which it has 

 undergone, resulting in the fixing of its characters, have made it so different in 

 many ways from the other members of the genus Harpoceras, that it cannot be 

 included in that genus without spoiling the compactness or upsetting the 

 definition thereof. 



I reserve any further remarks, and proceed to a description of the species, 

 when this matter will be further discussed. 



' TToXvTrXeKTos = closely-twined, in reference to the complicated sutures. 



