CLASSIFICATION CAMBRIAN BRACK lOPODA — WALCOTT 



151 



for microscopic examination was determined simply by close observa- 

 tion as grinding proceeded. Both vertical and tangential sections 

 were prepared, the former cutting the shell at right angles and the 

 latter cutting the shell in planes more or less parallel to the layers 

 or lamellae of which it is composed. The most interesting results 

 were obtained from the tangential sections, as the thin shells showed 

 little decided structure in vertical sections. 



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Fig. 2. Billingsclla plicaiclla Walcott [1905, p. 240]. Upper Cambrian, Gal- 

 latin Valley, Montana. 

 Diagrammatic sketch of a small portion of a tangential section, X 

 200. The granular ground-mass, with small pores and tubules 4 or 

 S times their own diameter distant from each other, is also typical 

 of other members of the Billingsellidse. 



Fig. 3. Dahnanella subequata (Conrad) [1843, p. Z2>2i]- Ordovician (Stones 

 River), St. Paul, Minnesota. 

 Photograph of a tangential section, X 35, showing the fibrous struc- 

 ture and comparatively large pores. 



Fig. 4. Kiitorgina cingulata (Billings) [x86i, p. 8]. Lower Cambrian, Swan- 

 ton, Vermont. 

 A small portion of the tangential section figured on Plate 12, fig. 4, 

 X 200. The minute structure of this and the following species is 

 essentially the same as that shown in fig. i, the only difference be- 

 ing the closer arrangement of the pores. 



Fig. 5. Obolus apolUnis Eichwald [1829, p. 274]. Upper Cambrian Obolus 

 sandstone, Esthonia, Russia. 

 Small portion of tangential section X 200. The minutely porous 

 granular structure is beautifully shown in this species, in which the 

 pores are arranged more closely than in any other observed. 



The general resemblance of the Cambrian eoorthoids to certain 

 Ordovician Protremata is so striking and the lines of descent so 

 suggestive that particular attention was devoted to this group, and 

 the examination brought out the fact that this apparent relationship 

 disappears when the shell structure of the two groups is compared. 



Sections of the shells of members of the Billingsellidae, of which 



