GRAMMOCERAS COSTULATUM. 197 



The Opalinum-zone (Mborei-beds) of Haresfield Hill has yielded the specimen 

 depicted in PL XXXIII, figs. 1, 2. Another specimen I obtained from Buckholt 

 "Wood on the same horizon. 



Grammoceras COSTULATUM (Zieten). Plate XXXIII, figs. 3, 4. 



1830. Ammonites costulatus, Zieten. Verstein. "Wiirtt., pi. vii, fig. 7. 



1858. — Aalensis, Quenstedt (non Zieten). Der Jura, pi. xl, fig. 10, 



only. 

 1879. Haepoceeas costula, Branco. Unt. Dogger; Abh. geol. Spez.-Karte 



Elsass-LothriDgen, Bd. ii, pi. i, fig. 9. 



1883. — Aalense, Wright. Monogr. Lias Amm. ; Pal. Soc, vol. 



xxxvii, pi. Ixxv, fig. 9, only. 



1884. — COSTULA, Wright. Ibid., vol. xxxvii, pi. Ixxxii, fig. 5, only (?). 



1885. Ammonites costula, Quenstedt. Amm. Schwabiscben Jura, pi. liv, 



figs. 7—14. 



Discoidal, compressed, carinate. Whorls broad, much flattened, ornamented 

 with distant, single, subarcuate radii, which soon die away, leaving the whorl 

 quite smooth. Ventral area undefined, acutely sloping to form a small, barely 

 distinct carina. Inclusion one-half. Inner margin slightly defined. 



The characteristic of this species is the decadence of the ribs after about one- 

 and-a-half inch diameter — that is, just the size of the specimen figured.^ The ribs 

 die away on the outer area first, and then finally disappear altogether. This 

 species is the direct descendant of Gramm. distans ; and it differs therefrom in 

 having smooth, broader whorls, sharper ventral area, and a smaller umbilicus. 

 Its inner whorls may be said to represent the distans-stage, but are somewhat 

 more involute. 



This species is also very like Gramm. aalense, which we need not wonder at, 

 considering how it is related by descent from the probable common ancestor, 

 Gramm toarcense ; but it is distinguished by the small number of regularly- 

 distanced, regular- sized ribs, set very widely apart from one another — by the early 

 age at which the test becomes quite smooth, or has only a slight appearance of 

 ribbing on the inner area- — by the greater sharpness of the ventral area. 



The horizon of this form is the Opalinum-zone {Moorei-beds) . It occurs at 

 Frocester Hill and Haresfield Hill, Gloucestershire, and although not so rare as 

 the last described species, is certainly very scarce. In PL XXXIII, figs. 3, 4, 

 two views of this species are given. 



^ My examples above this diameter are not good enough for delineation ; but a specimen may be 

 seen depicted in Dr. Wright's ' Monograph,' Pal. Soc, pi. Ixxv, fig. 9. 



