38 C. D. Walcott — Position of the Olenellus Fauna. 



Olenellus fauna, but, as shown by Brogger, in Sweden it is 

 more typical of the Middle Cambrian than of the Olenellus zone. 

 The undoubted species of the genus, known from the Olenellus 

 zone, are found in the upper portion, in association with types 

 of the Middle Cambrian fauna. The reference of Agnostus 

 nobilis Ford, from the lower part, to Agnostus is very doubt- 

 ful, as the form is probably a Microdiscus. The type which, 

 by a priori reasoning, should succeed Agnostus is Microdiscus, 

 with its three and four segments and eyeless cephalic shield. 

 As known, however, it occurs at the base of the Olenellus 

 zone, and its specific variations indicate prolonged existence in 

 a period of which the record has not yet been discovered. 

 Reaching its known maximum development, in species and 

 size, in the Olenellus zone, the genus diminishes in the Para- 

 doxides zone in about the same ratio that Agnostus increases 

 in importance. In the Upper Cambrian Microdiscus is repre- 

 sented only by Pemphigaspis bullata Hall. Agnostus continues- 

 on into the Middle Ordovician. Microdiscus connexus, of the 

 upper portion of the Olenellus zone, in New York, is the Para- 

 d oxides zone type of the genus, M.punctatus, while M. sculptus, 

 of the lower Paradoxides zone of South Wales, is the Olenellus 

 zone type, M. speciosus. 



The genus Olenellus has been found wherever the Lower 

 Cambrian fauna is known. It presents great variation in specific 

 characters, and I have included several of the species in the 

 sub-genus Mesonacis. The marked difference between this- 

 genus and Paradoxides is the absence of true facial sutures and 

 in the general configuration of the central portion of the head, 

 more notably in the form of the eye. Among the species of 

 Paradoxides the eyes of P. rugulosus Corda and the group of 

 species from the St. John terrane, of New Brunswick, approach 

 most nearly to those of Olenellus. In the type 0. Thompsoni, 

 the distinction between it and Paradoxides is very striking. 

 The absence of facial sutures and the long spine-like telson 

 finds no counterpart in the latter. 



0. {Mesonacis) Vermontana has the typical Paradoxides form 

 of pygidium, also a peculiar posterior series of thoracic segments 

 that are related to those of Paradoxides. This species appears 

 as a link between the type 0. Thompso?ii and the remaining 

 species referred to the genus, all of which have a pygidium 

 like that of Paradoxides, and none of the pleurae of the tho- 

 racic segments are prolonged, as in the type of the genus and 

 in the young of some species of Paradoxides. 



I would here call attention to the fact that while no true 

 facial sutures may exist in Olenellus there is, on the underside 

 of the test of the head, a line-like depression that corresponds 

 in position to the suture in Paradoxides. It may be well to 



