AEGOCERAS ARMATUM. 341 



description by Sowerby, who states^ that " numerous varieties of this species are found 

 in the great Alum-clay formations at Whitby, where this large-sized specimen was 

 gathered by Mr. Strangewayes. We have here also the advantage of many specimens, the 

 middle being a small plain one, which, indeed, might have been considered a diiferent 

 species ; the next circle might, by the same rule, form a second species with larger radii; 

 and, again, the third with the flat disks and fewer striae than the outer circles. It 

 is worthy of remark that the spines have the appearance of having been stuck on, pro- 

 bably owing to their being attached to part of the outer shell which is worn away at their 

 bases, the spines sometimes being gone also." In the Rev. George Young's ^ ' Geological 

 Survey of the Yorkshire Coast,' this author says " We have met with no shell corresponding 

 exactly with Mr. Sowerby's A. armatus, which he states to exist in our Alum-shale in 

 numerous varieties. We have several distinct species of knobbed Ammonites, but have 

 seen none with striated knobs stuck on like Barnacles or small Patellae, as in the outer 

 whorl of Mr. Sowerby's shell. We must therefore suppose that the latter is so rare, that 

 it could not be found among all the thousands of specimens which we have examined ; or 

 that Mr. Sowerby has been misinformed as to its locality ; or, which seems most likely 

 that he has made out his figure by combining two specimens together, of which the outer 

 one has not belonged to our Alum-shale, and the inner one has had some of its knobs 

 altered to connect it with the other." It appears from this passage that Mr. Young had 

 not seen a true A. armatus when he penned it, inasmuch as he figures in his pi. xiii, fig. 9, 

 one of the varieties of A. Jibulatus, described as " the inside of Sowerby's A. armatus 

 with a little variation in the knobs, which are not striated but plain and rather sharp. 

 The specimen appears to be nearly entire, and we have no reason to think that it has had 

 another whorl like that in Sowerby's figure. Yet in tracing the spire in its several 

 volutions, we see, as in his shell, a succession of different markings. Towards the mouth 

 the ribs are alternately knobbed and plain, and the knobbed rib parts into three at the 

 back, immediately beyond the knob, while the plain rib goes round the back single."^ 

 This confusion appears to have arisen from an error in Mr. Strangewayes' statement to 

 Sowerby that his specimen was gathered from the Alum-clay formation or Upper Lias, 

 whereas it was doubtless obtained from the lowest beds of the Middle Lias near the 

 village in Robin Hood's Bay, strata, by-the-by, which appear to have been very little 

 known when Young wrote his work, and so, failing to find A. armatus in the Alum- 

 shale, which he knew so well, he selected and figured as A. armatus a good spinous 

 variety of A. Jibulatus, a very characteristic Ammonite of the Alum-shale. This form he 

 supposed must have been the one Sowerby had before him when he very accurately 

 figured but imperfectly described an Ammonite which Young had not met with in the 

 Alum-shale, because it does not exist in that formation. Young does not appear to have 



1 'Mineral Conchology,' vol. i, tab. 95, p. 215, 1812. 



2 ' Geological Survey of the Yorkshire Coast,' 1st edit., p. 248, 1822. 



3 Ibid., p, 249. 



