GRAMJVIOCERAS ORBIGNYl. 185 



cannot, however, say if this be constant ; because, in what we can see of the 

 umbilicus of fig. 3, PL XXVII, there appears to be no such fasciation ; but as 

 the umbihcus is gone in the centre, I cannot make further researches. Also no 

 such fasciation is to be seen in the otherwise exactly similar specimen depicted by 

 d'Orbigny ; and that especially inclines me to the belief that this faint fasciation 

 was but a chance character. I therefore name the species Oramrri. Orbignyi. 



The differences between Gram/m. Orbignyi and the variety of Gramm. doerntense 

 are not very great. They may be summed up as greater compression, a slightly 

 wider umbilicus, a sharper ventral area and more acute carina, and a junction of the 

 ribs on the infier area on the part of the former. I have, however, observed that 

 there is some doubt concerning the constancy of the latter character. If constant 

 it might be thought to cause the species to resemble the type-form of Gramm. 

 doerntense^ but the coarser ribs and much squarer proportions of that form cause it 

 to be separated without diificulty. 



I fancy that this species is a descendant of Gramm. striatuluvi, from which — 

 barring the fasciation — it differs only by its broader whorls, finer ribbing, and 

 more elevated carina ; and therefore the development may be considered normal. 

 An alternative theory may, however, be held concerning its descent, and to this 

 I have alluded (p. 183). 



I consider it very curious that among the certainly large number of specim.ens 

 which I have collected from the Cotteswold Ammonitiferous deposits no specimen 

 of this species has rewarded my eiforts. The specimen now figured (PL XXVII, 

 figs. 5, 6) was sent to my father by the late Dr. Lycett as an example of his Am. 

 dispansus, and it bears a label in the doctor's handwriting, "A. dispansus, Lye. 

 radians, 'W.' Sands upper bed, Frocester Hill." 



It was evidently on account of the joined ribs that Lycett regarded this 

 specimen as a typical Am. dlspansus. As Lycett never figured his species there 

 might be some doubt as to which he really intended, but his statement^ that 

 "A. dlspansus .... probably equals in its numbers all other Ammonites at 

 Frocester Hill," is most characteristic of that species, and indicates that the 

 selection — as a typical Am. dispansus — of a specimen, which I have seen reason 

 to separate as a distinct species, was merely accidental. 



The possession of these fasciated ribs upon the inner margin might induce 

 the belief that this species was the parent of Gramm. dispansum ; but the 

 fasciated ribs certainly appear to die away too soon in Gramm. Orbignyi for that to 

 be the case. Another argument against this theory is furnished in the existence 

 of Gramm. metallarium ; and so we must, I think, regard the similarity of Gramm. 

 Orbignyi and Gramm. dispansum only as an instance of convergence. 



While writing of these two species I must not omit to mention how they differ, 

 1 ' Proc. Cotteswold Field Club,' vol. iii, p. 5, 1865. 



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