THE CAMBRIAN PERIOD. 



29^ 



of these were formerly placed under the genus Olenellus, but the last two are 

 now generally recognized as separate genera, and are highly characteristic of the 

 Lower Cambrian horizons. Holmia is found in the coast province, and in 

 Europe, notably in Sweden; it occurs rarely in the Appalachian province. 

 Mesonacis is found in both the Appalachian and coast tracts. Olenellus gilberti 

 (Fig. 126) is a typical form found in the western provinces. 0. Thompsoni occurs 

 in the Appalachian tract. The lowest order of trilobites (the Hypoparia) is repre- 

 sented by Microdiscus speciosus (Fig. 126, c). Its primitive characters are shown 

 in the apparent absence of eyes, the small number of thoracic segments, and its 

 similarity to the early stages of the higher trilobites. At least fifteen genera of trilo- 



Fig. 126. — Lower Cambrian Fossils. Trilobites: a, Mesonacis (Olenellus) vermontana 

 Hall; b, Olenellus gilberti Meek; c, Microdiscus speciosus Ford; Brachiopods: 

 d and e, Kutorgina cingulata Billings, side and dorsal or brachial views; / and g, 

 Iphidea labradorica, var. swansonensis Walcott, dorsal or brachial valve and cross- 

 section; i, Lingulella ccelata Hall; Gastropod: h, Platyceras primcevum Billings. 



bites, represented by fifty species, have already been found in the Lower Cam- 

 brian of America. Of brachiopods at least ten genera and thirty species are 

 known, and many more will doubtless be found. Among the characteristic forms 

 are Kutorgina cingulata (Figs. 126, d and e), Iphidea labradorica var. sirantonensis 

 (Figs. 126, / and g), Protorhyncha antiquata (Fig. 121, b), Obolella gemma (Fig. 

 121, a), Billingsella transversa (Fig. 121, e) and Acrotreta gemma (Fig. 121, c and 

 d), and Lingulella ccelata (Fig. 126, i). Of gastropods, at least a half-dozen genera 

 and a dozen species are known, among which were Stenotheca rugosa (Fig. 120, 

 c and d), and Platyceras primcevum (Fig. 126, h), characteristic forms. Amonn; 

 the typical pelecypods was Fordilla troyensis (Fig. 120, b). A quite common 



