298 CAMBRIAN BRACHIOPODA. 



Description of figures l-lSafi 



Billingsella plicatella Walcott [1905a, p. 240]: 



Figure 1. — Diagrammatic sketch of a small portion of a tangential section, X 200. The granular groundmass, 

 with small pores and tubules 4 or 5 times their own diameter distant from each other, is also typical of 

 other members of the BilUngselUdee. Upper Cambrian, Gallatin Valley, Montana. 



Dalmanella subequata (Conrad) [1843, p. 333]: 



Figure 2. — Photograph of a tangential section, X 35, showing the fibrous structure and comparatively large pores. 

 Ordovician, St. Paul, Minnesota. 



Kutorgina dngulata (Billings) [1861b, p. 8]: 



Figure 3. — A small portion of the tangential section shown in fig. 8, X 200. The minute structure of this and 

 the following species is essentially the same as that shown in fig. 5, the only difference being the closer 

 arrangement of the pores. Lower Cambrian, Swanton, Vermont. 



Obolus apollinis Eichwald [1829, p. 274]: 



Figure 4, — 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. Upper 

 Cambrian, Obolus sandstone, Esthonia, Russia. 



Billingsella coloradoensis (Shumard) [1860, p. 627]: 



Figure 5. — Photograph of horizontal thin section enlarged fifty diameters. This shows the characteristic granular 

 gi'oundmass of the Cambrian BillingselLidse. Upper Cambrian, Morgan Creek, Burnet County, Texas. 



Nisusiafestinata (Billings) [1861b, p. 10]: 



Figure 6. — Photograph of horizontal thin section enlarged fifty diameters. This section shows a granular ground- 

 mass in which faint indications of small pores or tubules may be seen with a high power. Lower Cambrian, 

 2 miles east of Swanton, Vermont. 



Eoorihis remnicha (N. H. Winchell) [1886, p. 317]: 



Figure 7. — Photograph of horizontal thin section enlarged fifty diameters. This section shows the same type of 

 groundmass as that illustrated by fig. 6. Upper Cambrian, Cold Creek Canyon, Burnet County, Texas. 



Kutorgina dngulata (Billings) [1861b, p. 8]: 



Figure 8. — Photograph of horizontal thin section showing granular shell substance. There are few slight indi- 

 cations of pores. Lower Cambrian, Swanton, Vermont. 



DaVmanella multisecta (Meek) [1873, p. 112]: 



Figure 9. — Horizontal thin section enlarged fifty diameters. This shows the fibrous structure of the shell pene- 

 trated by numerous fine tubules. Ordovician, Eden shale, Cincinnati, Ohio. 



Dalmanella parva (de Vemeuil) [1845, p. 188]: 



Figure 10. — Horizontal thin section showing fibrous structure; also section of the tubules that penetrate through 

 the shell. Middle Ordovician of Russia. 



SyntropUa lateralis (\Vhitfield) [1886, p. 303]: 



Figure 11. — Horizontal thin section enlarged fifty diameters, showing the arrangement of the pores in lines that 

 radiate from the apex toward the margin. Lower Ordtivician, Beekmantown, Fort Cassin, Vermont. 



Plectorthis plicatella (Hall) [1847, p. 122]: 



Figure 12. — Horizontal thin section enlai'ged fifty diameters. This section shows the fibrous structure so 

 characteristic of the Ordovician orthoids. Ordovician "Lowaine shaly limestone," Cincinnati, Ohio. 



Huenella abnormis (Walcott) [1905, p. 289]: 



Figure 13. — Horizontal thin section enlarged fifty diameters. The pores in this genus are smaller than in 

 Syntrophia, but their arrangement is essentially the same and shows the line effect characteristic of the 

 Pentameracea. Upper Cambrian, Gallatin Valley, Montana. 



Obolella crassa (Hall) [1847, p. 290]: 



Figure 14. — Horizontal thin section enlarged fifty diameters. This shows.the fine granular groundmass, with an 

 indication in the upper left side of the section that a surface ornamentation has been cut across. Lower 

 Cambrian, Bic, Canada. 



Obolus apollinis Eichwald [1829, p. 274]: 



Figures 15 and 15a. — Transverse vertical thin sections enlarged so as to show the lamellse and the presence of a 

 large tubule that appears to have more or less imperfectly penetrated through the shell. Upper 

 Cambrian, Obolus sandstone, Russia. 



a Figs. 1-4 are copied from figs. 2-5 on p. 151 of a preliminary paper on the classiiication of the Cambrian Brachiopoda [Walcott, 1908e]. Figs. 

 5-15 and 15a are copied Irom figs. 1-12, PI. XII, of the same paper. 



