parti] OF THE SHALES-WITH-' BEEF.' 81 



the other two forms in the character of its costation. Canavari l 

 included Dumortier's form in his ' jEgoceras'' deletion ; but, since 

 it is doubtful whether the Spezia ammonite is the same species as 

 the French, Fucini's name is here adopted. Pompeckj 3 wrongly 

 included Dumortier's much earlier form in iSchlotheimia lacunata 

 (J. Buckman), of the oxynotus zone. 



(H) Genus Microderoceras Hyatt. 



The earliest forms of this genus, apparently close to M. birchi 

 (Sowerby) but not well enough preserved to be definitely identified 

 therewith, occur at horizon 74 g, 18 feet below the main birch i 

 bed. About 18 inches higher, in 74 j, was found a crushed 

 Microderoceras (No. 4346) with two closely-approximating rows 

 of tubercles at a comparatively small diameter ; whereas, for 

 instance, the form figured by Quenstedt, 3 of which the mode of 

 preservation indicates the birchi (nodular) bed as the source, 

 has the outer whorl with two rows of tubercles close together, 

 but the inner whorls almost smooth to an exceptionally large 

 diameter. It is noteworthy, however, that forms apparently 

 indistinguishable from the badly-preserved example of bed 74 j 

 occur in both the birch /-nodular and the birchi- tabular beds. 



Specimens of Microderoceras from horizons 74 k, 16 feet below 

 the birchi (nodular) bed, and 74 m, 14| feet and 14 feet below 

 the birchi bed, also a more closely costate variety from 74 o, 

 10| feet below the birchi bed, are all too young or too badly 

 preserved for exact specific identification. In bed 74 r, 9 feet 

 below the birchi bed, the ammonites, including Arietites and 

 Cymbites, are again preserved in calcite, as in the birchi bed, 

 and not as crushed impressions in shales ; but the Microderoceras 

 are all small. They include one curious form having a single row 

 of median lateral tubercles at the diameter of 15 millimetres. 



Crushed impi*essions of Microderoceras occur again at distances 

 of 2 feet, 18 inches, 15 inches, and 14 inches below the birchi bed; 

 but the maximum development of the genus is in the main birchi 

 (nodular) bed. Whereas all the earlier forms of Microderoceras, 

 with the exception of fragments from 18 inches below this bed, 

 are small, the numerous specimens of M. birchi in the birchi- 

 nodular, including those figured by Sowerby, 4 Wright, 5 and 

 Reynes, 6 attain large dimensions. It has already been mentioned 

 that the only other ammonites in this JaVc/u'-nodular bed are 

 Cymbites and Arietites of the plotti group, although the latter 



1 ' Beitrsige zur Fauna des Unteren Lias von Spezia ' Palasontographica, 

 vol. xxix (1882) p. 166 (44). 



2 ' Beitriige zu einer Revision der Ammoniten des Schwiitrischen Jura ' 

 pt. i, Jahresh. Ver. Vaterl. Naturk. Wiirtt. vol. xlix (1893) p. 237. 



3 ' Ammoniten des Schwabischen Jura ' 1883, pi. xviii, fig - . 1. 



4 ' Mineral Conchology ' vol. iii (1820) p. 121 & pi. cclxvii. 



5 ' Monogr. Brit. Lias Ammonites ' Pal. Soc. 1882, p. 332 & pi. xxiii. 

 8 ' Monographic des Ammonites ' 1879, pi. xxxviii. 



Q. J. G. S. No. 313. a 



