C. D. Walcott — Position of the Olenellus Fauna. 39 



note that Olenellus resembles the living Limulus in having 

 well developed eyes, without the presence of facial sutures. 

 The external resemblance to Limulus is further enhanced by 

 the telson-like pygidium of O. Thompsons. The structure of 

 the cephalic appendages of the trilobite also relate it to Limu- 

 lus.* 



If we consider the head of Limulus to belong to a more 

 highly organized form than the head of Paradoxides, the fact 

 that the head of Olenellus is without facial sutures does not 

 make it rank below Paradoxides. In fact, Olenellus JBroggeri, 

 of Newfoundland, impresses me as being as highly organized 

 as any of the species of Paradoxides, if not more highly. 

 Olenellus Thonvpsoni and 0. OiTberti might be considered the 

 progenitors of Paradoxides, inasmuch as they have a strong 

 development of the pleura of one of the thoracic segments, a 

 feature that is present in the young of P. Bohemicus, but does 

 not continue in the adult. 



American paleontologists have considered the genus Olenel- 

 lus as the descendant of Paradoxides, but the fact of occurrence 

 proves such a theory to be incorrect. The argument advanced 

 by Mr. Ford,f that the young of Olenellus asa/phoides passed 

 through the Paradoxides stage, in its embryonic development, 

 may be explained in another way, by assuming that the species 

 of Olenellus, having the pleurae of the third segment prolonged 

 (macropleural), originated earlier than those with the pleurae of 

 uniform length (brachypleural), and hence the prolonged 

 pleurae are shown only in the embryonic phases of growth in 

 the brachypleural species. As pointed out b} 7 Mr. Ford, the 

 genus Paradoxides, like Olenellus, has brachypleural and ma- 

 cropleural species, but it is significant that it is in the young 

 of Paradoxides that the macropleural feature of 0. Thomp- 

 sons is developed, while in the adult it is reduced to a very 

 insignificant character. That an intimate if not a genetic rela- 

 tionship exists between Olenellus and Paradoxides there is 

 little doubt. Olenellus exhibits greater specific variation and 

 is more diversified by spines on the head and thorax, but in 

 the essential elements of structure it is very closely related to 

 Paradoxides. With the exception of 0. {Mesonacis) Vermon- 

 tana there are no known connecting species between the typi- 

 cal forms of the two genera. 



The genus Olenoides is largely developed in the Middle 

 Cambrian of the interior of the continent. One species only, 

 0. Marcoui, is found in the lower portion of the Olenellus 

 zone. Two other species, O. Fordi and 0. quadriceps, are in 

 the upper portion, near the passage between the Lower and 



*Bull. Mus. Comp Zool. Harvard College, vol. viii, No. 10, 1881, pp. 208-211. 

 f This Journal, III, vol. xx'ri, 1881, pp. 250-259. 



