Article: Ontogeny of the trilobite Peltura scarabaeoides from Upper Cambrian, Denmark
Poulsen's original material of the developmental stages of Peltura scarabaeoides is redescribed. It includes protaspides of length 0.4 mm. and cranidia or cephala up to a length of 1.6 mm. The protaspid glabella is elliptical in outline, widest at the mid-length, and divided into five rings. In meraspid cephala of length greater than 1 mm. the glabella is widest across the occipital ring and glabellar furrows are lateral. The outline of the anterior cephalic margin changes from forwardly concave to forwardly convex. In a cephalon of length 1 mm. (including occipital spine) the posterior branch of the facial suture crosses the cephalic border immediately out-side the base of the fixigenal spine. In a smaller cranidium and the protaspis the exact course cannot be seen, but it may be similar. The fixed cheeks become narrower relative to the occipital ring as size increases, in contrast to their development in other olenids. Fixigenal and median occipital spines are present in a cranidium of length 1.3 mm., but not in larger cranidia. The ontogeny of the cephalon of P. scarabaeoides is, among olenids, most like that of Leptoplastides salteri. It was claimed by Poulsen that a considerable migration of the posterior branch of the suture occurred in small cephala of P. scarabaeoides. Such a migration cannot be demonstrated, and theoretical deductions based in part on this claim are now open to doubt.