420 S. S. Bach man — On Jurassic Ammonites. 



VI. — Notes on Jurassic Ammonites. 

 By S. S. Buckman, F.G.S. 



TWO valuable pamphlets on Jurassic Ammonites have lately 

 appeared. One of these is entitled " Cephalopodes 

 nouveaux ou peu connus des Etages jurassiques de Normandie, 

 par Louis Brasil." ' This author, who lias paid very considerable 

 and successful attention to the Jurassic rocks in the neighbourhood 

 of Caen, describes and figures (there are four plates) nineteen new 

 species of Ammonites belonging to fourteen genei-a, viz. : Lytoceras, 

 Pliylloceras, Catulloceras, Zurcheria, Dorsetensia, Bajocia, PceciU>- 

 rnorphus, Hammatoceras, Erycites, Oppelia, Caclomoceras, Slrigoceras, 

 Cosmoceras, CEcoiyphius. Of these genera one is new, namely, 

 Bajocia, founded on an unique specimen ; and it must be confessed 

 that the example is sufficiently distinct in every way to 

 warrant this treatment. Although the author does not say so, 

 it is evident the genus belongs to the subfamily Sonninise ; and 

 M. Brasil practically admits as much when he states that it is 

 very near to Dorsetensia. He further saj's that he considers it 

 to be a branch from Baploplenroceras, whose representatives have 

 lost not only the two rows of spines, but even the keel and the 

 furrows. We find ourselves unable to agree with this opinion, 

 because the periphery is rounded and flattened ; in our experience 

 the keel and furrows do not disappear without leaving some trace 

 of their former existence on the configuration of the periphery. 

 We agree with the author that Bajocia is near to Dorsetensia, and 

 we are inclined to think it is a late-surviving example of the stock 

 whence Dorsetensia originated, and a " cousin " of Zurcheria. 



Besides the new species, M. Brasil is able to give us descriptions, 

 and in some cases figures, of previously described forms; and in all 

 this work he adds to our knowledge of Jurassic Ammonites, and 

 deserves our thanks. 



We find that M. Brasil has carried into effect a suggestion which 

 we made a few years ago. We wrote (" Bajocian of the Sherborne 

 District": Q.J.G.S., vol. xlix, p. 483) concerning the use of the 

 name Humpliriesianus for chronological purposes — " The confusion 

 concerning this specific name and the large number of species of 

 Stephanoceras make this appellation vei-y unsuitable. It would be 

 desirable to apply the name of a species belonging to a less prolific 

 genus." M. Brasil makes very similar remarks concerning the 

 confusion, and then says — " It appears to us that Dorsetensia 

 Edouardiana ought to replace Ammonites Humpliriesianus." Whether 

 this will free us from the confusion, or will only introduce another, 

 remains to be seen. We remember that, unfortunately, our own 

 identification of D. Edouardiana has been challenged, whether 

 rightly we will not pretend to say, without an extended comparison 

 of our specimens with the actual type, because it seems that the 

 original figure is not altogether reliable. 



In this matter of draughtmanship have arisen most of the troubles 



1 Extrait clu Bulletin de la Societe geolog-ique de Normandie, tome xvi, 1892-3 

 (published 1896). 



