part 4] JURASSIC chro:n^ology : lias. 303 



•mineral condition, as he has obtained it in situ (Avhich fixes the 

 horizon for the first specimen), and, as it marks a definite horizon 

 in the Kaasayan, a full name is desirable. 



This species is comparable .with two of G-eyer's, but differs from 

 his jEgoceras 'pr<jecursoi\ with which it shares the radial linear 

 ornament, by whorl-shape — whorl widest at the periphery — the 

 retention of the inner row of spines, and yet having less whorl- 

 inflation. His jEgoceras hispinaticmi^ separable by having a more 

 rounded peripher}^, a different whorl-shape, more approximate 

 spinicostse, and less differentiation^ in spines of the two rows. For 

 references and proportions, see table below. 



History of the types. — Holotype, from a bed of Lias yielding 

 pyritized fossils in the neighbourhood of L^^ne Eegis (Dorset) — 

 purchased there. Paratype, ' in place, bed 94, about 4 feet above 

 Oxynotus bed. About 800 yards west of Westhay Water, east of 

 Chkrmouth,' Dorset ; Coll. W. D. Lang, No. 2578. See Table V, 

 facing p. 267. 



Table XVIII. — Proportions : Deroceras, etc. 



Beroceras hispinigerum, holotype S. 5.5, 28, 35, 50. 



Do. do. paratype S. 37, 28, 34, 49. 



^ r. ».^ rF.48, 32, 38, 45, 



jEgoceras praecursor (jrejev '■ i T 50 26 30 49 



^goceras hispinatum Gejer^ F. 55, 30, 35, 46. 



Genus Microderoceras Hyatt. 

 MiCRODEROCERAS DEPRESSUM, nom.nov. (PI. XXX,figs. la & lb.) 



1867. Ammonites hirclii Damovtier {non J. Sovvevby), depressed variety, ' Bassin 

 du Rhone ' vol. ii, pi. xli, figs. 1, 2, p. 130. 



Remarks. — This name applies to the shell figured by Dumortier, 

 which he notes as different from that figured by Sowerby and 

 A. d'Orbigny. He calls it a depressed variety, and that appellation 

 is retained in the present trivial name. The holotype, however, is 

 an English specimen Avhich agrees with Dumortier's shell, except 

 in one respect — Dumortier's shows peripheral radii (costulae) nearly 

 straight and parallel ; in the English shell the radii connecting 

 tubercles are nearly straight, but the intermediaries have rather 

 more forward bending. 



The stouter whorls with a more pronounced umbilical wall, a 

 slightly shorter smooth stage, and a small increase in the number 

 of tubercles to a whorl in middle age, are features of distinction 

 from Ammonites hirclii. 



Proportions are given below (p. 306). 



History of the specimens. — Figured specimen (No. 1280) 

 from blue Lias, excavation for reservoir of Bristol Waterworks, 

 Barrow Grurney (Somerset), presented by Mr. J. W. D. Marshall. 



^ ' Ceph. Hierlatz ' Abhandl. k. k. Geol. Reichsanst. vol. xii (1886) pi. iii, 

 fig. 27. 2 j5^^_ pi^ i^^ flg.^ 4^ 



