Notes on Thecal Variation in Monograptus

Abstract

Thecal modifications in certain Monograptids are discussed on a basis of restorations which distinguish the theca from the remainder of the “thecal segment” of Törnquist. There seems little support in those bi-form Monograptids examined for the view that the less modified thecae reflect an incomplete development, and in general the new thecal types appear to represent transformations which commonly originate proximally or at least are more highly developed proximally. A possible analogy between this colonial development and metamerism is suggested.