360 S. S. Buchnan — On Jurassic Ammonites. 



Cymbites stern alis (von Bucli). 



1843. Ammonites sternalis, d'Orbigny, Ceph. Jurass. Pal. franc;, pi. 111. 



1885. Ammonites sternalis, Queustedt, Amiu. Schwabischen Jura, pi. 50, figs. 6 



and 7. 

 1893. Paroniceras sternale, Bonarelli, snl Toarciano, etc., p. 202 (10). 



Von. Buch's work is not at present accessible, so that my remarks 

 must be considered to refer to the above-quoted figures, D'Orbigny's 

 figures appear to indicate that the carina was phyletically nascent in 

 this species — a carinate periphery being, in this case, a developmental 

 advance upon one uncarinate. 



Upper Lias. Zone ? 



Cymbites subcarinatus (Young and Bird). 



1822. Nautilus subcarinatus, Young and Bird, Geology of Yorkshire Coast, pi. 



xii. fig. 7. Also 1828, ed. 2, pi. xii. fig. 9. 

 1829. Ammonites subcarinatus, Phillips, Geology of Yorkshire, pi. xiii. fig. 3. 



(The figure is really not identifiable.) 

 1862. Ammonites subcarinattcs, Oppel, Pal. Mitth. pi. 44, figs. 1 and 2. 

 1876. Phylloceras subcarinatum, Blake, Yorkshire Lias, p. 297. 

 1884. Phylloceras subcarinatum, Wright, Lias Ammonites Pal. Soc. pi. 81, 



figs. 1-3. 

 1893. Paecilomorphus subcarinatus, BonarelK, Osserv. sul Toarciano; Boll. Soc. 



geol. italiana, vol. xii. fasc. 2, p. 5. 



The subcarinate periphery of sternalis has developed into the 

 bisulcate-carinate periphery of suhcarinatus ; the striae have become 

 ribs, and the small umbilicus has become larger ; these are all pro- 

 gressive developmental features seen in analogous series. 



This species appears to be well distributed, but not common. It 

 occurs in Yorkshire, in the Midlands, and in the South of England. 

 At Shepton Beauchamp, in Somerset, I found it in situ, just at the 

 top of the beds, with Harpoceras falciferum — in fact just at the 

 place where Harpoceras falciferum and Hildoceras hifrons came into 

 contact. 



The position of the two species, sternalis and subcarinatus, has 

 always seemed to be a puzzle to paleontologists, ' yet their relation- 

 ship to the globosus series appears obvious from their septation and 

 general characters. They differ only in their peripheral aspects ; but 

 this is merely a matter of development. When an Ammonite- 

 periphery departs from the arched shape, it becomes either elevated 

 or depressed medianly. In the former case its full phases of 

 development are, in consecutive order, fastigate,^ subcarinate, 

 carinate, carinate-bisulcate. These changes can only become of 

 generic value when the phyletic series splits into two stocks — one 

 of which retains, say, the arched periphery, the other acquires a 

 carinate periphery. 



Now Cymbites is, in my opinion, the radical of all Animonaceee ; 

 and, unlike Hyatt, who goes to planorbis for his radical, I consider 

 that globosus-like Triassic species — perchance Nannites — are the 

 parents of our Lower-Lias Ammonites. From Cymbites the various 

 Ammonite-stocks have been cast off at difi'erent times. Thus 



^ "Forms very doubtful as to their position." — Neumayr, Unverm. Ceph. p. 67. 

 ^ Fastigatus, rising evenly, like the roof of a house ; sloping. 



