52 W. H, Hudleston — On the Yorkshire Oolites. 



with which a group of shells not altogether uncomraon in the Dogger 

 and the Dogger Sands could in any way be identified. The third 

 edition ignores the existence of this group in the Dogger entirely, 

 and the question as to how far the common Cerithiiim of the Coralline 

 Oolite is identical with certain forms in the Dogger is thus quietly 

 shelved. 



No one doubts the practical identity of the Steeple Ashton fossil 

 with the ordinary forms of G. muricatum, so abundant in the Coral- 

 line Oolite at Pickering : yet it is not easy to indicate how these 

 diifer from the Dogger types ; even the varieties being pretty much 

 the same. The Coralline Oolite specimens from Pickering have 

 rather a wider spiral angle. It is true that in the Dogger Sands are 

 forms pi-esenting considerable differences, and it may be a legitimate 

 question whether all of these should be included here, but there can 

 be no doubt, with the specimens before us, that the difficulty as to 

 the Dogger types disappears, and that, whatever stratigraphical 

 palaeontologists may say to the contrary, Sowerby was justified in 

 placing the shells from the Peak and from Steeple Ashton under 

 the same specific designation. 



General Description. — Since modifications of the ornaments take 

 place from absorption, from deposition of foreign matter, or from 

 other causes due to differences of conservation and of the physical 

 character of the matrix, it is important to seize upon such points as 

 are least liable to be modified by mineralization. The following 

 may be accepted as the general diagnosis of the CeritMum muricatum 

 group in well-developed specimens. 



Shell turrited : length, usually 18 — 25 mm. ; spiral angle 16° — 

 22° ; whorls moderately tumid, flat in the middle, constricted to- 

 wards the base, and subangular in general outline, 12 — 14 in number, 

 width of whorl to height as 5 : 3-5. Suture well marked : ornaments 

 conspicuous, consisting of spiral bands varying from 3 to 6 in num- 

 ber, which decussate with more or less numerous longitudinal ribs : 

 the latter are frequently arcuate and have a slight inclination from 

 left to right.^ Nodes or spinous granulations occur at the points 

 of intersection of the bands and ribs, and these nodes are drawn out 

 spirally, in some cases more than others, but always spirally. The 

 upper spiral band on each whorl is the most salient, producing the 

 muricated character of the spire, though this feature is by no means 

 confined to the group under consideration : the other spiral bands 

 are equal or nearly so. The spiral belts in the base are strongly 

 marked but plain. 



Some of the varieties occurring in the Lower Oolites are partly 

 represented by C. granulate and 6'. muricato-coslatum, Miinst. (Goldf., 

 1844, pi. 173, figs. 10 and 12), whilst Romer's Turritella quaclrilin- 

 eata (Ool. Geb. p. 154, pi. 11, fig. 14), which is quoted by that 

 author from the Posidonienschiefer of Hildesheim, is without doubt 



^ N.B. — It may be worth remarking that in the Chemnitzia vetKsta group the 

 longitudinal ornaments are more conspicuous than the spiral ones : in the Geriihiuni 

 muriciUum group the two systems of ornament are about equally strong, whilst in 

 the Turritella of the Dogger the ornamentation is altogether arranged spirally. 



