164 SMITHSONIAN MISCEILIvANEOUS COLLECTIONS VOL. 53 



DESCRIPTION OF PLATE 12 



Billingsella coloradoensis (Shumard) [i860, p. 627] : 



Fig, I. Photograph of horizontal thin-section enlarged fifty diameters. 

 This shows the characteristic granular ground-mass of the Cam- 

 brian Billingsellidse. Upper Cambrian, Morgan Creek, Burnet 

 County, Texas. 



Nisusia festinata (Billings) [1861, p. 10] : 



Fig. 2. Photograph of horizontal thin-section enlarged fifty diameters. 

 This section shows a granular ground-mass in which there are 

 faint indications of small pores or tubulse which may be seen 

 with a high power. Lower Cambrian, 2 miles east of Swanton, 

 Vermont. 



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



Fig. 3. Photograph of horizontal thin-section enlarged fifty diameters. 

 This section shows the same type of ground-mass as that illus- 

 trated by fig. 2. Upper Cambrian, Cold Creek Canyon, Burnet 

 County, Texas. 



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



Fig. 4. Photograph of horizontal thin-section showing granular shell sub- 

 stance. There are few slight indications of pores. Lower 

 Cambrian, Swanton, Vermont. 



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



Fig. 5. Horizontal thin-section enlarged fifty diameters. This shows the 

 fibrous structure of the shell penetrated by numerous fine 

 tubules. Ordovician 'E,den formation, Cincinnati, Ohio. 



Dahnanella parva (de Verneuil) [1845, p. 188] : 



Fig. 6. Horizontal thin-section showing fibrous structure ; also section of 

 the tubules that penetrate through the shell. Middle Ordovi- 

 cian of Russia. 



Syntrophia lateralis (Whitfield) [1886, p. 303] : 



Fig. 7. Horizontal thin-section enlarged fifty diameters, showing the ar- 

 rangement of the pores in lines that radiate from the apex 

 toward the margin. Lower Ordovician Cassin limestone. Fort 

 Cassin, Vermont. 



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



Fig. 8. Horizontal thin-section enlarged fifty diameters. This section 

 shows the fibrous structure so characteristic of the Ordovician 

 orthoids. Ordovician Lorraine shaly limestones, Cincinnati, 

 Ohio. 



