Rev. J. F. Blake — On Ammonites. 165 



allows when young the same peculiarity of the periphery, and 

 whose sutures, as drawn in the adult, show the same kind of 

 development as one might expect from the smaller examples, 

 provided that both figures really belong to one species. With 

 ■regard to Sowerby's figure on tab. 293, fig. 1, matters are plainer : 

 it represents, as already stated, the typical form of the Yorkshire 

 fossil known as Am. plicatilis, acknowledged to be so from the 

 intended representation of it in pi. iv, fig. 29 of the 3rd edition of 

 the " Geology of Yorkshire," revised by E. Etheridge ; it corresponds 

 also to Sowerby's description of A. plicatilis of tab. 166, which, as 

 already noted, so far agrees with that of tab. 293, fig. 1. It was 

 for this reason I supposed Sowerby's specimens had probably been 

 interchanged, being guided by Agassiz's translation of his work, 

 but Miss Healy has drawn attention to the character of the matrix, 

 which I had entirely overlooked, which puts an end to this idea 

 and at the same time opens up new considerations. By no possibility 

 could any fossil in such a matrix be found in any bed at Dry 

 Sandford, nor in any of the Corallian beds at Headington, The 

 fossil in itself is, however, perfectlj^ normal, but it has been 

 separated septarially along a calcite-filled crack running principally 

 nearly parallel to the median plane. This has raised the upper 

 surface and separated the lower, as pointed out tome by Mr. Crick ; 

 but the small central portion is quite continuous with the outer 

 whorls, on the upper side at least. 



But the problem is, where to find a septarian matrix containing 

 a Corallian fossil. Looking over all the fossils referred to Corallian 

 or Oxfordian strata in the British Museum, one only was noted with 

 a similar, very similar, matrix. It was the matrix of 'Am. varicostatus,' ^ 

 and the locality given was " Hackleton," which is in a drift-covered 

 -district about 5 miles from Northampton towards Bedford or 15 miles 

 from Hawnes. My own purchased specimen, locality unknown, 

 but horizon stated as " Oxford Clay," and perfect to the centre, has 

 also been preserved in a septarium.- The specimen of Sowerby's 

 pi. 293 has evidently been knocked out of a similar rock, and the 

 second fragment has a similar matrix.^ On the other hand, we may 

 naturally look for such specimens in localities where Corallian rocks 

 are represented by clays, and especially where septarian doggers 

 are recorded as occurring. Such are found near the summit of the 

 ■Corallian clays at Ampthill (see Woodward, " Jurassic Rocks of 

 Britain," vol. v, p. 136). 



From these considerations we may safely conclude that the fossils 

 figured as A. biplex, but usually called A. plicatilis, are the inside 

 whorls, ver}'^ likely broken out of the middle, of larger specimens 

 called A. varicostatus. The latter retain the old age characters, 

 though such characters are common to several species. There 



^ The spelling- of Bucklaud, probably au oversight, as corrected by Phillips. 



^ Supposed at first to be from Osmington, but this shows that it was not so. 



3 After the proof aiforded by Mr. Crick of the Cornbrash age of Nautilus truncntus, 

 stated by Sowerby to be from the Lias of Keynsham, we cannot place too much 

 reliance on the localities given by the latter. 



