84 INFERIOR OOLITE AMMONITES. 



descendant from Ps. compactile, and these specimens approach in shape somewhat 

 nearer to the parent form than Schloenbach's figures do.) 



The fact that this genus occurs at about the same horizon as Harpoceras 

 falciferum would seem to preclude any idea of its being descended from the genus 

 Harpoceras. Its state of development, too, prevents this conclusion, because it 

 is, in my opinion, more developed in some respects and less developed in others 

 than is the genus Harpoceras ; or, in other words, it differs from the parent stem 

 more in some ways than Harpoceras and less in others. 1 It also stands just in 

 the same relation to the genus Lioceras, which first appears at about the same, or 

 even at a somewhat lower, horizon. 2 It is not, therefore, the parent of Lioceras, 

 and, being less developed, it can scarcely be its descendant. The fact is that we 

 have in the Upper Lias three distinct stem-forms derived from some common 

 ancestor which existed prior to the Margaritatus-zone. The descendants of two 

 of these three stem-forms are traceable into the Inferior Oolite as parallel genetic 

 series. Doubtless to this cause we owe that convergence of certain species 

 among them — species belonging to different genera — which makes the separation 

 and definition of genera and species an intricate task. When, however, we are 

 able to trace the different species back to their parent forms, we can recognise 

 and appreciate the reasons for their separation. By inducing close observation 

 of the characters of the various species, this mode of generic subdivision should 

 become of considerable assistance and benefit to such researches. 



According to my interpretation of Ammonite-evolution, this genus is the 

 parent of the next genus Hyperlioceras, and is itself descended from Am. Gurioni, 

 Meneghini, of the Margaritatus-zone. Commencing in the Upper Lias, it under- 

 went certain modifications in course of time, and finally died out with the 

 Opalinum-zone, having given birth (probably through Ps. compactile) to the forms 

 in the Concavum-heds which, because they possess certain characters peculiar to 

 themselves distinguishing them from their parent genus, I have placed in a 

 genus Hyperlioceras. 



1 See pp. 134, 138, 139. 



2 Prof. Blake quotes Lioceras elegans {Harpoceras concavwn, 'Yorkshire Lias,' p. 303) from the 

 zone of Am. annulatus, Harpoceras falciferum (his Harp, serpentinum) from the so-called Serpentinum- 

 zone, and Ps. lythense {Harpoceras lythense, p. 304) from the zone of Am. communis. Dr. Wright 

 quotes the last {Harp, lythense, ' Monogr. Lias Amm.,' p. 445) from the Serpentinum- {i. e. falciferum) 

 zone. 



