164 CLASS III. GASTEROPODA. ORDER VII. PECTINIBRANCHIATA. 
agrees with it in every character of form, &c., has a solid calcareous one ; 
the Turbo pica, again, has a horny operculum, whilst the Turbo sarmaticus 
has a calcareous one. 
To avoid confusion, therefore, we preserve the genera Trochus and 
Turbo in their integrity, relying upon the practicability of dividing the 
species with tolerable accuracy according to the shape and formation of 
their shells. The columellar tooth, by which Lamarck distinguished his 
Monodonte, is of such wide and uncertain application that we have aban- 
doned that genus as unnecessary, in imitation of Sowerby and Deshayes, 
and we restore the genus Phorus of De Montford, for the reasons already 
given in treating of it. Other genera proposed by this author, Calcar for 
the Trochus stellaris, Tectus for the Trochus maculatus, Cantharidus for the 
Trochus iris, and Clanculus for the Trochus Pharaonis, and those allied to 
it, we cannot so highly appreciate. 
The shell of Trochus may be described as being conical, pyramidal, and 
flattish underneath ; the spire is elevated, and the whorls, which are rather 
numerous, have their periphery more or less acute ; the columella is curved, 
and often truncated or dentated at the base, sometimes it is crenated ; the 
margins are disjoined, and the lip is sharp, and either simple or denticu- 
lated. The operculum is sometimes horny, sometimes calcareous. 
Examples. 
PLIC@XVAl. Figs! 
Trocuvs TRISERIALIS, Lamarck, Anim. sans vert., vol. vil. p.22. De- 
lessert, Recueil de Coquilles, pl. 35. f. 7. a, b. 
Pl. CCXVII. Fig. 2. 
Trocuus catcar, Chemnitz, Conch., vol. v. p. 48. pl. 164. f. 1554 to 
1557. 
Pl. CCXVII. Fig. 3. 
Trocuus asteriscus, Nobis, Proceedings Zool Soc., 1842. 
Pl. CCXVII. Fig. 4. 
Trocuus unposus, Wood., Supp. Index Testaceologicus, pl. 5. f. 1. 
