128 INFERIOR OOLITE AMMONITES. 



b. Hammatoceras, Hyatt emend. Haug. 



a. Gruppe des Hammatoceras insigne. 



/3. ,, ,, Sowerbyi. 



y. Untergattung Sonninia, Bayle emend. Haug. 



c. Oppelia, "Waagen. 



II. Arietitidj:. 



Nachkommen der Arieten mit Sarpoceras-Gepratge. 



Hildoceras, Hyatt emend. Haug. 



a. Untergattung Lillia, Bayle em. Haug. 



/3. Hildoceras s. str. 



•y. Untergattung Ludwigia, Bayle em. Haug. 



7 1 Gruppe des Ludwigia Murchisonce. 



•y 2 ,, ,, hectica. 



■y 3 ,, ,, ignobilis. 



In a later paper, 1 however, Dr. Haug has somewhat modified this arrangement, 

 and has expressed the opinion, formed on an examination of a greater series 

 of specimens, that the groups of H. radians, H. kurrianum, H. falciferum, 

 H. lythense, are closely connected with Arietites, and have nothing to do with 

 Mgoceras. 



If the idea be correct that the Hectici are derived from Ludwigia, and Ludwigia 

 from Lillia which is closely allied to Hildoceras, it follows that the separation of 

 Ludwigia from the Arietites-like forms, as Zittel has done, will not hold good. Dr. 

 Haug derived Ludwigia first from Hildoceras ; 2 but he afterwards 3 changed this 

 opinion and derived it from the group of Harpoceras falciferum, though at the 

 same time expressing his belief that they are all descendants of Arietites. 



The question which we have now to answer is, Are all the species of Zittel's 

 two divisions of Harpoceras descended from some common ancestor, say Arietites, 

 and, if so, what relationship do they bear to one another ? Before we can give a 

 correct answer we must examine the groups which Zittel has made. Groups 

 a, b, c of Division I most certainly fall together very naturally, but d and e do not 

 appear to me to belong to them. 4 In Division II we have a slightly more com- 

 plicated state of things ; a falls, rightly enough, to Grammoceras ; b is the genus 



1 " Ueber die genetischen Beziehungen der Gattung Harpoceras" ' Neues Jahrbuch fur Mineral - 

 ogie, &c.,' Bd. ii, 1885. 



2 " Beitrage Monog. Harpoceras." 3 Op. cit. * See p. 133: 



