| dc.description |
The characters of the lateral furrows of the glabella and of the pits which occur in the axial furrows of the head-shield have not had sufficient attention paid to them, and a comparative study of their development in various British species reveals some interesting and important features.It has heen commonly assumed that lateral furrows are absent in some species, and at any rate the omission of any notice of their occurrence in certain cases is difficult to understand. Frequently, moreover, it has been stated that only two pairs are present, even in those species in which the presence of any lateral furrows has been recognized. The nature of the so-called stalk or neck of the glabella has been also generally misunderstood. Mccoy's division of the genus Trinucleus into two genera, Trinucleus, sens, restr., and Tretaspis, was largely based on the presence or absence of this stalk and of lateral furrows; for he says the furrows at the base of the glabella distinguish Tretaspis from Trinucleus, the latter being stated to be without lateral furrows, though the presence of the ‘eye-line’ on the cheek was chosen as the primary distinctive feature of the former. Nicholson and Etheridge, while rejecting Tretaspis as a sub-generic division, mention an additional third pair of furrows at the sides of the anterior swollen portion of the glabella in Trinucleus Bucklandi, though they do not seem to regard them as segmental furrows. |
