J. Starkie Gardner — On the Gault Aporrhdidce. 129 



and there are no ribs on the last two whorls ; the canal is very long, 

 considerably exceeding the length of the spire. 



The Blackdown specimens present a somewhat different aspect, 

 and the description of this species, to apply to them, has to be 

 modified in the following manner : — there are seven to eight or nine 

 whorls, the spire varies from an angle of about 22° to 33°, and is 

 sometimes pupasform, but with the apex drawn out and tapering 

 very gradually. The last two or three whorls form an obtuse 

 termination in exactly the same manner as the Folkestone shell, but 

 this termination is far less conspicuous, and only to be seen in well 

 preserved specimens, or by the aid of a lens, as the spire tapers to 

 a finer point, and the apical whorls are therefore smaller. The 

 mode of growth is the same as that of the Folkestone forms, but 

 nothing could be more variable than the number and distinctness of the 

 ribs. These are sometimes entirely obliterated, leaving only faint 

 traces in crossing the keels (Fig. 8), 1 sometimes they are very pro- 

 nounced and regular (Fig. 1). On the last whorl in joarticular, the 

 ribs are sometimes wholly absent, in other cases they extend to the 

 anterior keel, being bent in their passage over the median keel, 

 giving them an angularly flexuous appearance (Fig. 15). There are 

 strongly marked varices on some of the shells. The wing is shorter, 

 broader and stronger, but when perfect is always produced in an 

 acute, upward point : at the point of curvature it is broad, with 

 even, though very rarely, a tendency to become bifurcated (see 

 Fig. 16). The aperture is the same as in the Folkestone shells, but 

 the anterior canal is never long. 



These differences would be by many, as they have been by Prof. 

 Morris and others, considered sufficient to constitute the Blackdown 

 fossils into a separate species ; but frequent examination of a large 

 series of specimens has convinced me that they cannot be so sepa- 

 rated, as I am utterly unable to find any fixed specific character by 

 which to distinguish them ; the more ordinary forms of this shell 

 from both localities being all but identical with each other. I shall 

 allude farther on again to these differences, and hope to offer some 

 explanations which may help to account for them. 



Distribution. — It is exceedingly abundant both at Folkestone and 

 Blackdown, and not uncommon at Shanklin. On the Continent it is 

 found at Ervy, Courtaout, Dienville, Cosne, etc., and specimens may 

 be seen in most museums. 



History. — Parkinson first figured this species as a Bostellarite in 

 the Organic Bemains, vol. iii. p. 63, pi. v. fig. 2. Sowerby in the 

 Min. Conch, vol. iv. p. 69, pi. 319, figured and described it very 

 carefully, naming it R. calcarata; and D'Orbigny in 1842 gave an 

 enlarged figure of this shell from the Gault of Ervy, Pal. Fr. Terr. 

 Cret. pi. 207, fig. 3. 



I am not quite certain whether the figures and descriptions of 

 A. Muleti, composita, etc., of other authors are identical with our 

 species, as their shells are considerably larger, specimens in the 



1 Mr. Ealph Tate, Geol. and Nat. Hist. Eepertory, Sept. 1865, named this variety 

 " neglecta." 



DECADE II. — VOL. II. NO. III. 9 



