372 THE LIAS AMMONITES. 



Robin Hood's Bay ; it very much resembles one of the forms of Aeg. Jmnesoni, var. sagitta- 

 rium, in the cabinet of the Rev. J. E. Cross, F.G.S., and may indeed prove to be such, but 

 in the absence of more specimens for comparison I shall retain the name I gave it long ago. 



It is a regular, well-formed Ammonite, with rounded whorls, which are only slightly 

 involute. The ribs, which are sharp and narrow with a sigmoidal flexure, arise from a 

 thickened root. At the spiral suture they wind up the side and over the margin (fig. 1), 

 and vanish in the middle of the siphonal area (fig. 3), which is convex and smooth ; 

 near the turn of the penultimate whorl (fig. 2j the ribs are sometimes enlarged before 

 they terminate, leaving only a narrow smooth space in the middle of the area. The 

 umbilicus is wide, and the inner whorls are all fully exposed. The aperture is oblong 

 and a little flattened at the sides. 



The lobe-line (fig. 7) is apparently simple ; the siphonal lobe is longer and wider 

 than the principal lateral, and has a few digitations with two terminal points. The 

 siphonal saddle is small and terminates in three small leaves. The principal lateral 

 lobe is narrow and oblique with four digitations on each. The lateral saddle is wide 

 and much larger than the siphonal, it terminates in three chief foliations and several 

 smaller ones. The lateral lobe is larger than the principal lateral, with three lateral on 

 each side and two terminal digitations. The auxiliary saddle has a foliated termination, 

 and the auxiliary lobes are Small with oblique digitations. 



Affinities and Differences. — This Ammonite resembles some of the transition phases in 

 the evolution of Aeg. sagittarium. I know of no other form with which to compare this 

 Cephalopod, which remained long unnamed in my old friend's collection, and is now 

 figured and described for the first time. 



Aegoceras Portlockii, Wright, nov. sp. PI. XLVIII, figs. 4, 5, 



Diagnosis. — Shell discoidal, compressed, and carinated ; whorls compressed, slightly 

 involute, thicker at the spiral suture, and tapering towards the keel ; sides ornamented 

 with stout regular ribs, having a well-defined sigmoidal figure, and being well rounded 

 throughout ; carina stout, into which the ribs appear to blend, the space between the ribs 

 concave and smooth. 



Dimensions. — I have only the fragment of a whorl. 



Description. — This fragment was collected by the Irish Geological Survey, and 

 represents a form which I have from the Aeg. Jamesoni beds of Robin Hood's Bay. The 

 whorls are wide and slightly involute ; they are covered with strong, regular rounded 

 ribs, having a sigmoidal flexure, and all obliquely placed at regular intervals apart, and 

 concave intermediate spaces between them. The carina is thick and prominent, and the 

 ribs, slightly enlarged before they terminate, appear to blend into the keel. The specimen 



