Vol. 54.] DIVISIONS OP SO-CALLED ' JURASSIC ' TIME. 449 



of the Polymorphidae. With the disappearance of the Dumortierice 

 the Harpoceratan Age is fittingly brought to a close. 



Here I would pause to say a few words concerning the Harpo- 

 ceratan Age, on account of my treatment of its stratigraphical equi- 

 valent Toarcian [Stage] in a former paper * On the Cotteswold, 

 Midford, & Yeovil Sands.' ^ At that time, treating the term 

 Toarcian in a somewhat different way — namely, from a mixed 

 stratigraphical and palaeontological point of view — I proposed that 

 the Toarcian, during which the Hildoceratidae were dominant, should 

 include all the strata which were deposited during what are now 

 called Harpoceratan and Ludwigian Ages, divided, however, into 

 Lower Toarcian from falciferum to opalinum, and Upper Toarcian 

 from Murchisonce to concavum (see p, 473 of that paper). 



Now, the dominance of the Hildoceratidae throughout the Harpo- 

 ceratan and Ludwigian Ages is even more fully confirmed to-day 

 than when I wrote the paper just quoted, owing to our better 

 knowledge of the affinities of ammonite-species. But this predomi 

 nance suffers interruption during two hemerse, when Dumortierice 

 of the family Polymorphidae hold nearly undisputed sway. If 

 this interruption were to be disregarded, and the term Harpo- 

 ceratan (time of Toarcian Stage) were to be applied to the whole 

 space of time dominated by the Hildoceratidae, the result would be 

 an Age nearly twice as long as any of the others ; and this would 

 be highly undesirable. Therefore it is adviBable to take note of the 

 interruption in the dominance of the Hildoceratidae, and to consider 

 this event as a chronological landmark. As in the case of the 

 Deroceratan Age ending with the incursion of the Amaltheidae, so 

 it is advisable that the Harpoceratan Age should have similar treat- 

 ment, and be regarded as coming to a close with the incursion of 

 the Bumortierice. Consequently the Ludwigian Age should begin 

 when the Hildoceratidae resume their sway. By this means the 

 two Ages, Harpoceratan and Ludwigian, are brought to be of 

 duration compatible with the length of time occupied by other Ages. 

 (d) The Ludwigian Age consists of six hemerae, and it is domi- 

 nated almost entirely by the Hildoceratidae — a fact uniting it closely 

 to the Harpoceratan Age. Further the Hildoceratidae are, of all 

 ammonite-families, the nearest allied to the Arietidae of the Sine- 

 murian ; and thus the biological features of the Ludwigian Age are 

 similar to those of the preceding Ages. Noticeable during a portion 

 of this Age is another attempt on the part of the Deroceratidae to 

 become established, in the case of Hammatoceras and Erycites ; these, 

 however, never become of any considerable specific importance 

 compared with the Hildoceratidae. But the close of the Ludwigian 

 Age is marked by the influx of another series of Amaltheidae, 

 namely the Sonnininae. 



As to the close of the Ludwigian Age, it is not continued in the 

 present arrangement to include the hemera disciUje, wherein the 

 final disappearance, so far as we know at present, of the Hildo- 



1 Quart. Journ. Geol. Soc. vol. xlv (1889) p. 440 



2x2 



