part 4] EVOLUTION OF THE LIL'AROCERATIDjE. 275 



sutures from the third to the tenth are almost identical in each 

 species, the second suture having a ventral lobe which becomes 

 divided by a minute median saddle in the next suture (text-fig. 5 c, 

 p. 274). The first signs of den tic ulat ions appear in JEc/oceras 

 latcecosta at a diameter of 3o mm. and by 45 mm. the external 

 saddle has three unequal terminal cells ; the relatively-small first 

 lateral lobe is feebly trifid (text-fig. 5d, e, p. 274). This lobe 

 remains narrower than the external saddle but becomes deeper, 

 and is as deep as the external lobe by the time that a diameter of 

 9 mm. is attained. The features of the suture may now be said 

 to have reached the proportions characteristic of the adult. 



In many young examples of this species the suture is tilted ; 

 that is to say, the saddles of one side are in advance of those of 

 the other. If, therefore, only one side of such a specimen is 

 •examined, its sutures appear to resemble those of either Amblyco- 

 ■cras or Beclieiceras. 



iEGOCEEAS .EQEICOSTA, Sp. 110 V 



Compare JEgoceras maculatum Wright (8, pi. xxxiv, figs. 1 & 3). 

 Dimensions of holotype : — 



Diameter. Whorl height. Whorl thickness. Umbilicus. 



70 mm. 28 per cent. 26 per cent. 45 per cent. 



Whorl slender, almost circular in section, with uniformly-spaced 

 ribs, little diminished on the periphery, which they cross without 

 any curve. Thirty ribs on the outer whorl, where they are rounded ; 

 "the ribs of the inner whorls, like those of AE. latcecosta, are more 

 acute. Suture of typical AEgoceras form, with large external 

 saddle and narrow first lateral lobe. 



AEgoceras cequicosta is proposed to include the English am- 

 monites which were formerly called Ammonites capricornus 

 Schlotheim, a name which is not used here, since its precise 

 application is not known. 



[^Occurrence. — Type-specimen, from the latcecosta sub-zone of 

 Lincoln. 



JEgoceras cequicosta differs from other capricorns in 



(1) Nearly round whorl (the outer whorl of M. latsecosta is more 



elevated). 



(2) Eibs very little diminished on the periphery (distinguishing it from 



most capricorns without a peripheral rib-curve). 



With yEgoceras cequicosta at Lincoln occur specimens which 

 s'dw a catagenetic subcostate-striate stage. 



In the latcecosta sub-zone of Dorset Dr. Lang has found 

 ammonites, presumably of this lineage, which show a more advanced 

 stige, resembling A. Tienleyi Sowerby in the moderately-round 

 whorl, with flattened sides, and it is suggested that these 

 ammonites (W. D. L. Coll. Xo. 3861) are intermediate between 

 AS. latcecosta and A£. dcedalicosta. They have swollen whorls, 



