262 



MR. T. ROBERTS ON THE UPPER JURASSIC ROCKS 



These 7 species are found in the Astartian of the Jura, together 

 with the following Lower-Kimeridge-Clay forms : — 



Strophodus reticulatus, Ag. 

 Natica inicroscopica, Contj. 

 Pleuromya tellina, Ag. 

 Area rhomboidalis, Contj. 



Astarte supraeorallina, D' Orb. 

 Terebratula Gesnerij Et. 

 Oidaris spinosa, Ag. 



From these lists it will be seen that the Lower-Kimeridge-Clay 

 fauna is well represented in the Astartian of the Jura. As stated 

 above, 15 Corallian species also occur in these beds, and some of 

 them range up into the Pterocerian ; but these fossils must be re- 

 garded rather as the dying-out of the Corallian fauna than as indi- 

 cating any affinity of the Astartian of the Jura with the Corallian of 

 England. It must be remembered that the Astartian beds are 

 formed principally of limestones, with but little argillaceous admix- 

 ture, and the absence of the latter material would be favourable 

 to the prolonged existence of some portion of the Corallian fauna 

 of the Jura ; whilst in England the Corallian fauna for the most 

 part disappeared at the introduction of the great clayey period 

 at the close of Corallian times. Admitting this to be the case, 

 the Astartian must be regarded as representing part of our Lower 

 Kimeridge Clay and Kimeridge Passage-beds. Blake makes the 

 beds with Ostrea deltoidea &c, at the base of the Lower Kimer- 

 idge, the upper limit of the Astartian in England ; but there 

 appears to be some evidence for including a larger portion of our 

 Lower Kimeridge in the Astartian. Astarte supracorallina has been 

 found in the Jura only in the Astartian beds, and this fossil, or at 

 least a form which is very hard to distinguish from it, occurs 

 fairly commonly in the Lower Kimeridge Clay of England, and has 

 also been recorded from the Sandsfoot Clays and Grits (Supracoral- 

 linebeds of Blake and Hudleston). In Cambridgeshire the following 

 zones have been made out in the Lower Kimeridge Clay (Sedgwick 

 Essay for 1886, MS.) :— 



4. Clays crowded with Eocogyra virgula. 

 3. Clays with Ammonites alternans. 

 2. „ „ Astarte supracorallina. 

 1. „ „ Ostrea deltoidea. 



The fossil characteristic of zone no. 2 has not, as yet, been 

 found in this district above that horizon. If the range of Astarte 

 supracorallina, which is characteristic of the Astartian, be of any 

 value in indicating the limits of the equivalents of the beds in 

 the two areas, the clays containing this fossil in England should 

 certainly be included in the Astartian. The other Lower-Kimeridge 

 fossils which are present in the Astartian of the Jura also support 

 this view. 



The fauna of the Astartian beds in the southern district of the 

 Jura has much in common with that of the northern district, and a 

 comparison of their fossils with that of the Lower Kimeridge of 

 England points to much the same conclusion. 



