JURASSIC FOSSILS. 379 



fere with the lines. The form of the different lobes and saddles cannot be 

 obtained from any of the specimens of moderate size in the collection, they 

 being- too much mutilated, so Ave have copied the following description of 

 these parts from the authors as above cited : 



"The dorsal lobe is a little wider than long, and has two principal 

 branches on either side, the two terminal of which are slightly larger than 

 the others, and each provided with seven or eight unequal digitations. The 

 dorsal saddle is about the size of the superior lateral lobe, contracted in the 

 middle, and divided at the extremity into some four or five short, unequal, 

 sinuous, and digitate branches. The superior lateral lobe is as long as the 

 dorsal lobe, but narrower, conical in form, and ornamented with three or 

 four lateral branches on the dorsal side, and two or three smaller ones on 

 the ventral side ; while its terminal branch is bipartite, and its margins, as 

 well as those of all the other principal divisions, are more or less sinuous 

 and digitate. The lateral saddle is smaller than the superior lateral lobe, 

 and has on each side three or four short, obtuse subdivisions, with sinuous 

 margins. The inferior lateral lobe is smaller than the lateral saddle, and 

 divided at the extremity into two nearly equal, rather short branches, each 

 of which is sinuous, and shows a disposition to give off short subdivisions 

 on the outer side. The remaining lobes are very small, and obtusely digi- 

 tate, the inner one showing a tendency to bifurcate." 



The species is exceedingly variable in its surface markings, and also 

 somewhat so in the degree of convexity, the latter feature depending mostly 

 on the age of the specimens ; but the former variations are individual char- 

 acters only, and are almost limitless among the individuals even from the 

 same locality. The smaller specimen figured on Plate 6 has the primary 

 costa numerous and crowded, and seldom dividing or increasing to more 

 than twice the original number on approaching the dorsal margin ; while 

 the next larger individual figured has them more distant at the umbilicus, 

 and dividing or increasing to the number of six to one in some parts, while 

 on still other specimens many or most of the costa are obsolete. 



Messrs. Meek and Hayden compare this species with A. cordatus 

 Sowerby, and appear to be somewhat in doubt if it should not be referred 

 to that very variable form. There are, however, differences in the form of 



