TROCHOCERATID^. 
33 
Trochoceras cornu-arietis, J. de C. Sowerby, sp. 
1839. Lituites cornu-arietis, J. de C. Sowerby, in Murchison’s Sil. Syst. 
pt. ii. p. 643, pi. XX. f. 20, pi. xxii. f. 18 (varieties a & j3). 
1843. Lituites cornu-arietis, var., Portlock, lleport on the Geology of 
Londonderry, p. 383, pi. xxviii. b. f. 7. 
1849. Lituites Sowerbianus, d’Orbigny, Prodr. de Pal6ont. Stratigr. 
vol. i. p. 1. 
1852. Lituites cornu-arietis, M‘Coy, British Pal. Foss. fasc. ii. p. 323. 
1855. Lituites cornu-arietis, Salter, ibid. Appendix A, p. viii. 
? 1860. Lituites cornu-arietis, Eichwald, Lethaea Rossica vol. i. Seconde 
Section de I’ancienne Periode, p. 1298. 
1882. Lituites cornu-arietis, Blake, British Foss. Ceph. pt. i. p. 217, 
pi. xxi. ff. 6, 6 a ; pi. xxviii. f. 5 (Portlock’s figured specimen). 
1845. Lituites cornu-arietis, de Verneuil, in Murchison, de 
Verneuii, and de Keyserling’s Geol. de la Russie d’Europe, vol. ii. 
pt. iii. p. 359, pi. XXV. fif. 7 a, &.] 
Sp. Char. The description of the type specimens by Professor 
Blake runs as follows ; — “ There are two so-called varieties of this 
species, according to Sowerby, and they have apparently somewhat 
different characters. In the example, var. a, which is merely an 
external cast, the rate of increase is 1*42, last whorl *33 of the 
diameter. The several whorls are just in contact, all exposed, fi.t- 
ting closely to the centre, and not quite symmetrical ; they are 3 
in number, and there is no straight portion. Section apparently 
rounded ; thickness yL the whorl-breadth. The ribs are backward 
curving, not clearly separate, becoming wider apart with age, and 
covered by parallel riblets. Diameter 16 lines. From Caradoc 
Sandstone, Gorton, near Presteign. In the other example, var. /3, 
the rate of increase is only 1*2, and the last whorl *3 of the whole, 
80 that the inner whorls are partially concealed. It is now com- 
pressed, and thus has an acute front [convex side], but the sides 
are uniformly convex. The ribs are sharp, and about 27 per whorl, 
curving backwards towards the convex side, and are covered by 
parallel riblets. No septal characters are seen. From the Lower 
Llandovery Shales. Both the specimens are in the Museum of the 
Geological Society.” 
Professor Blake further observes that “ in all the collections ex- 
amined [by him] there are but few examples of either of these forms, 
and they do not range themselves definitely in two groups as though 
there were two species, but are most satisfactorily considered as be- 
longing to one rather variable one.” A specimen figured by Blake 
shows that the ornaments change with growth. “ In the first 1|- 
whorls they are merely fine riblets, only occasionally rising to a 
PART II. n 
