21. The lower boundary is taken at the transition from red continental shales of the Lunde formation to the overlying sandstones. 22. The lower boundary of the Cambrian was originally held to represent the first appearance of complex life, represented by trilobites. 23. The lower boundary of the Floian is defined as the first appearance of " Tetragraptus approximatus ", a graptolite species. 24. The Ergilian's lower boundary is the approximate base of the Priabonian age and approximate upper base of the Rupelian age. 25. The arms lie on an unseen parapet coinciding with the lower boundary of the painting, according to the Flemish painting tradition. 26. It is very likely that further investigations will push back the Sri Lankan lower boundary to match that of South India. 27. The lower boundary coincides with the ground ( or, more precisely, a height above ground corresponding to its aerodynamic roughness ). 28. Earth may in fact lie on the lower boundary of habitability : if it were any smaller, plate tectonics would be impossible. 29. Landslips often occur at or close to the lower boundary of the Tunbridge Wells Sands, between it and the underlying Wadhurst Clay Formation. 30. The Tabenbulakian's lower boundary is the approximate base of the Chattian age and upper boundary is the approximate base of the Aquitanian age.