38 TYPE AMMONITES— VI Dec. 



these, so that the gap is from the Shell Bed to the rotundum beds— 

 in the Isle of Purbeck, according to Blake, some 240 feet. 



Dr. Salfeld (Jura Nordwesteui;opa ; N. Jahrb. Beil.-Bd. xxxvii, 

 1913. p. 208) says : " Im ' White ^.eptarien-Band ' fand sich ein partiell 

 verdriickter Perisphinctes aus der Gruppe des P. Pallasianus D'Orbignv." 

 This gives a hint as to where to look for the equivalent of the Hartwell 

 Clay fauna, for quite possibly this is near to Ammonites kimmeridiensis , 

 Seebach, which must be noticed. 



Ammonites kimmeridiensis, Seebach, 1864, (Hannoversche Jura, 

 p. 157). The author gives this name to Am. biplex, " Damon Suppl. 

 to the geol. of Weymouth Taf. 10, Fig. g " — a misprint for Taf. g, fig. g. 

 Damon's figure, which is thus protograph of Seebach's species, is now 

 reproduced in T.A. DCLXXIII. Damon says it comes from Portland 

 Stone and Kimmeridge Clay, so that two species are involved — one of 

 which is, possibly, akin to " Perisphinctes " gorei. Seebach says (p. 156) 

 that it is only known to him from the Kimmeridge ; but, as his Kimmer- 

 idge strata are beds below the ^/gas-schichten (p. 157), evidently a third 

 species is involved. This shows how unreliable for correlation are these 

 so-called biplex ammonoids, unless they are very critically determined. 



It is now suggested that Damon's form, which obviously has not 

 gorei ribbing, is actually from Kimmeridge Clay, and is what Salfeld 

 calls a Perisphinctes of the P. pallasianus type from the White Septarian 

 Band (see above, p. 37) — that it is, in fact, a Holcosphinctes, possibly 

 little different from the later-named H. pallasioides, Neaverson. 



Had Damon said that this species came from the Portland Sand, 

 it might have been suggested that it came from the Blue Cement 

 Stone, and was very near to a Crendon specimen (T.A., PI. DCLXXVl) 

 from the Waterstone, which should be about the horizon of the Hounstout 

 bed. This Crendon specimen is a fourth complication and fourth date 

 for the so-caUed biplex forms. It is distinct from Lydistratites, 

 Pallasiceras and other forms by lacking the periodic thick ribs which 

 border constrictions : it shows neither character. 



A biplex ammonoid, presumably- by matrix from the Blue Cement 

 Stone, is quite like .immonites skidegatensis, Whiteaves (Mes. Foss. i 

 (i) ; Geol. Surv. Canada, 1876, p. 34, PL ix, i, not PI. vii), in outer 

 whorl, but the Hounstout specimen (No. 4650, S.B. Coll.) has more 

 crowded ribbing in the inner whorls. 



The White Septarian Band is some 200 feet below the top of the 

 rotundum beds, and about another such amount above the ' Oil Shales,' 

 judging by Blake (Kim. Clay; Q.J.G.S., xxx, 1875, p. ig8). It has been 

 claimed that the Oil Shales represent the Shotover Grit Sands of 

 Oxfordshire, but they have not yet yielded the necessary evidence 

 of characteristic ammonoids, many of them giants not easily overlooked — 

 Paravirgatites, Shotoverites, Wheatleyites. All the evidence given is 

 small forms of very uncertain characters, and the biplex ammonoids 

 have shown how similar forms can be repetitive. But if the claim Avere 

 made good, it would be equivalent to saying that at Shotover there is 

 a gap of something like 300 feet between the Shotover Grit Sands and 

 the immediately overlying Littleworth Lydite Clay. 



The present correlation scheme, which suggests that there is no 

 important non-sequence at Shotover at such place, that the Littleworth 

 Clay succeeds the Shotover Sands without noticeable break, while the 

 representatives of the Shotover Sands and the Hartwell Clay are to be 

 sought between the rotundum beds and the Oil Shales of the Dorset 



