Article: Sphaeroidal enrolment and thoracic characters in Beltella depressa and other olenid trilobites

In sphaeroidal enrolment, the exoskeleton formed a closed capsule. Well known in post-Cambrian trilobites, many Cambrian species, in addition to Agnostina and eodiscoids, also enrolled in this manner. Characteristic features of such forms (other than Agnostina and eodiscoids) are: a fulcrate thorax; the fulcrum relatively close to the axis; the facets of the anterior segments large and backwardly directed; more posterior facets smaller, steeper, and less so directed. These features are described in detail in Beltella depressa and Peltura scarabaeoides scarabaeoides, and were shared by other olenids, including Triarthrus, in which sphaeroidally enrolled specimens are known. Other olenids, in which the fulcrum was distant from the axis, may have enrolled in the cylindrical manner, which left a lateral gap in the enrolled exoskeleton. The significance in classification of these and other characters of the thorax has yet to be evaluated in Olenidae ar other groups.