SIXTH, OR OXYXOTICERAX BRANCH. 219 



of this subseries. Quenstedt's Amm. Guibalianus is not a true Greenoughi, and is 

 properly referred by him to the radians group. An error is also shown by the 

 use of this name for a species of the Middle Lias, since Guibalianus does not 

 occur above the Lower Lias. 



The species as figured by Wright is a large shell, about 475 mm. in diameter, 

 which has already lost the keel and pilae, the abdomen being rounded. The 

 internal whorls of the original are heavily pilated, but the last whorl and a half 

 are smooth. Sowerby's figure is taken from a very large and aged specimen, and 

 the pilations shown in the umbilicus indicate a shell with heavier folds at the same 

 age than exist in either Hauer's or Wright's figures. Wright's descriptions, how- 

 ever, coincide with Sowerby's figure, and the old of Sowerby's shell has a com- 

 pletely elliptical aged whorl. The size at which senility affects the shells, and 

 the general characters and aspect of these figures, are the same as in Greenoughi. 

 The young forms figured by Hauer may possibly be the young of this species ; 

 they seem to be too heavily pilated for the young of oxynotum. 



Oxynoticeras Guibali, Hyatt. 



PI. X. Fig. 38, 39, 31. Suinm. PI. XIII. Fig. 14, 



Amm. vicloris, Dom., Etudes Pal. Bass, du Rhone, II. p. 136, pi. xxxi., xlii. 

 Amm. Guibali, Reyn^s, Plates (pars). 

 Amm. Guibaliayms, Reynes, Ibid. 



Locality. — Lyme Regis? 



The keel of this species may begin to disappear in some specimens even at the 

 size of 100 mm. In one specimen of the Semur collection this is accompanied by 

 a singular and marked lateral deflection of the hollow keel, and at the size of 

 170 to 180 mm. it had wholly disappeared, and the outer whorl had a very broad 

 and gibbous abdomen ; the sides, however, remained convergent and rounded. 



There is a specimen of this species from Lyme Regis in the Museum of Com- 

 parative Zoology, which was found associated in the same slab with Gal. carusense 

 and raricostatum. The diameter of this specimen is 145 mm. The keel is not 

 present on the cast, as is usual in this species, but it had probably been present 

 in the perfect shell, and evidently not impaired by age ; the. whorl and invo- 

 lution of the sides were also not altered from the adult condition. A section 

 showing the inner whorls had been formed by fracture, and the outlines of these, 

 the hollow keel, and the amount of involution of the whorls, are the same as in 

 Oxyn. Guibali. 



Amm. vicloris, Dum., is evidently closely allied to Greenoughi, but, as figured 

 by Dumortier, presents very peculiar sutures. It attains a large size before 

 losing the keel, though one was seen by Dumortier which at a diameter of 

 456 mm. had no keel. It may prove to be a form which is transitional between 

 Greenoughi and this species. 



