Researches on the Visual Organs of the Trilobites

Before proceeding further I shall give a general representation of the shape of the hypostoma. This enigmatic part of the trilobite skeleton resembles as to its outline a heraldic shield being broad at its anterior margin, gradually tapering towards the posterior extremity, which generally is acuminated or, as in the Asaphidæ, bifid. In a few, as Deiphon, Remopleurides, it is nearly square. Its exterior surface may consist of a single field or he divided transversally into two or three fields, according to the presence of one or two shallow grooves.