HARPOCERAS OPALINUM. 465 



inner lateral, and the other two auxiliaries are much smaller, with saddles of corre- 

 sponding sizes (see PI. LXXX, fig. 5). 



Affinities and Differences. — This shell resembles Ilai-p. radians (PI. LXXXI, figs. 4, 

 5) in possessing biflexed striae and a sharply carinated siphonal area, but is distinguished 

 from that species by the absence of a distinct keel, and in having smaller and less prominent 

 striae on the sides, a much narrower umbilicus, and more numerous side-lobes in the 

 septa. It still more closely resembles Harp. Aalense (PI. LXXX, figs. 1, 2), from which 

 it differs only in a very few particulars and these are as follows. The whorls are higher, 

 they have always an angular and concave inner margin (PI. LXXX, figs. 4, 6) ; the 

 umbilicus is narrower; the inner whorls are almost entirely concealed, the striae are more 

 marked and flexed, and the side-lobes more numerous. Harp, opalinum and Harp. 

 Aalense are doubtless very nearly allied forms, and which apart might be mistaken for 

 each other, but when carefully compared the distinctions pointed out appear to be 

 persistent. The carefully drawn delineations of Harp. Aalense, which I have given, and 

 which I have placed on the same plate for comparison with those of Harp, opalinum, will 

 aid the student in his diagnosis of Harp, opalinum and Harp. Aalense much better than 

 words. 



In reference to this species I annex the following remarks sent to me by the late Dr. 

 J. Lycett : 



" In my paper entitled ' Notes on the Ammonites of the Sands intermediate to the 

 Upper Lias and the Inferior Oolite ' ^ I have followed the example of all preceding palaeon- 

 tologists and arranged Ammonites opalinus as distinct from Am. Aalensis, Ziet. An 

 examination of numerous Giindershofen specimens oi Am. opalinus has recently convinced 

 me that the two supposed species are identical, and that both are referable to varieties of 

 Ammonites Aalensis described in the paper above referred to. 



"In the course of the twenty years which comprise my experience of collectino- at 

 Frocester Hill, it has often been a matter of surprise to me that no specimen of Am. 

 opalinus should have rewarded my endeavours, and that only some two or three 

 examples should have been obtained by other collectors at that locality ; it now appears 

 that some examples of the variety of Am. Aalensis termed by me Moorei differ from Giinder- 

 shofen specimens in no feature of any importance, and that the greater envelopment of 

 the volutions exhibited by the majority of the foreign specimens is by no means per- 

 sistent even in them, the Cotteswold specimens exhibiting much variability in this 

 respect, some few of the more depressed forms having the volutions much enveloped, 

 and acquiring some degree of angularity upon the inner slopes of the volutions, as is 

 more commonly seen in the Giindershofen shells ; nor are instances wanting in the latter 

 wherein the radii become large, distant, irregular, and unequal in size, thus constitutin<y 

 the variety 'Aalensis;' upon the whole, however, the Giindershofen specimens of all 

 sizes do not exhibit so considerable a variability in the figure and ornamentation as is 

 1 'Proceedings of the Cotteswold Nat. Club,' vol. iii, p. 1, 1865. 



60 



