82 MIDDLE LIAS. 



they constantly maintain an unvariable relative position." It was from the upper shales 

 of this zone at Robin Hood's Bay that Luidia MurcJiisona, Will., was obtained, and 

 from beds of the same age at Skinningrave that Plnmaster oj^hiuroides, Wr., was extracted. 

 It would appear, therefore, that the northern limit of this stage in England is characterised 

 by species of AsTEiiiADyE entirely distinct from those found in the south, although the 

 species of the molluscan fauna are the same throughout. 



On the Dorsetshire coast, as near Charmouth, this zone forms the upper portion of 

 the gray micaceous marls, or " the green Ammonite-bed" of local collectors. The Char- 

 mouth form of A. ca-pricornus, Schl., was figured by Sowerby in his ' Mineral Conchology,' 

 vol. vi, pi. 556, fig. 2, as A. latcecosta ; and to increase the existing confusion regarding 

 the identity of this species, a mistake was made in the text in reference to the 

 figures — A. lafacosta was described as A. brevisjnna, and vice versa ; this fact I have 

 verified by an examination of the original specimens. A. latcecosta. Sow., is therefore 

 only a variety of A. cajoricornus from the upper portion of the gray marls, whilst A. 

 brevispina, Sow., is a distinct species, which I have hitherto only detected among fossils 

 from the zone of Ammonites Jamesoni at Pabba. In my description of Ammonites 

 brevispina, Sow.,^ it is stated — " Sowerby's text and the numbers of pi. 556 do not agree ; 

 this mistake will mislead the reader, unless he is acquainted with the two Ammonites 

 figured in that plate, for fig. 1 represents Amm. brevispina, Sow., and fig. 2, A)nm. 

 latacosta, Sow. ; the former is a Pabba, the latter a Charmouth, fossil. 



Ammonites Loscombi, Sow., is a very abundant form, and which has a very wide range 

 throughout the gray micaceous marls. 



11. The Zone of Ammonites margaritatus. 



Synonyms. — " Marlstone and ironstone series," Phillips, ' Geol. of York.,' p. 192. " The 

 Marlstone," Murchison, ' Geol. of Cheltenham,' 2nd edit., p. 37. " Marlstone," William- 

 son, "Fossil Remains on the Yorkshire Coast," 'Geol. Trans.,' 2nd ser., vol. v, p. 224. 

 "Marlstone," Hull, 'Memoirs of the Geol. Survey,' descrip. of sheet 44, p. 18. "Lias 

 Delta, Amaltheenthon," Quenstedt, 'Plozgeb.,' p. 540. "Marnes "k Ammonites amaltheus 

 ou margaritatus" Marcou, ' Jura Salinois,' p. 50. " Macigno d'Aubange," Dumont, 

 Dewalque et Chapuis, 'Luxembourg,' p. 273. "Die oberen Schichten des Ammonites 

 maryai'itatus,'' 0])^q], ' Juraformation,' p. 133. " IMargaritatus-bed," Wright, 'Quart. 

 Journ. Geol. Soc.,' vol. xv, p. 25. 



The Marlstone forms a well-known division of the Lias formation. In Gloucestershire 

 it consists, according to Mr: Hull,^ of two parts — the lower, a scries of yellow, gray, 

 and brown sands, with thin bands of calcareous limestone and ferruginous nodules; the 



1 'Quart. .Journ. Geol. Soc.,' vol. xiv, p. 2(S. 



2 'Memoirs of the Geol. Survey. The Country around Cheltcnliam,' p. 18. 



