ORDER I 



ATRKMATA 



805 



of growth, the austral group (.l/^/r/A//,/m>r) passes through a different series 

 of loop metamorphoses from the boreal one (Dallinae). 



It was by the application of the above-mentioned prinnpk-s that Srhuchcrt. 



in 1893, arranged all the genera of Brachiopoda under the four orders 

 instituted by Beecher. Further attention has since lu-cn -i\en to this subject 

 by the same writer, and the classification outlined in his SynOptM of Aim rin< 

 Fossil Brachiopoda has been adopted in the present work. 



Order 1. ATREMATA. Beecher. 



Inarticulate Br<n-hi<>i'<nhi with the pedicle emerging freely between the two valves, 

 the opening being more or less sha.ml !>// Imfli. Growth taking place mainly around 

 the anterior and lateral margins, never enclosing or surrounding the pedicle. Aperture 

 unmodified. Prodeltidium attached to dorsal valve. 



Superfamily 1. OBOLACEA. Schuchert. 



Rounded or semi-circular, and more or less lens-shaped, thick-shelled, primitive 

 Atremata, fixed by a short pedicle throughout life to extraneous objects. Cambrian 

 to Silurian. 



Family 1. Paterinidae. Schuchert, 



Obolacea with nearly semi-circular valves without cardinal areas, 

 ami with the entire, gaping, posterior region occupied by the pedicle. 

 This is the simplest shelled condition of the class, and is repeated 

 ontogenetically as the protegulum, or initial shell, probably through- 

 out the class. Cambrian. 



Paterina, Beecher (Fig. 505). Of this North American 

 genus, the only one of the family, about six species are 

 known. 



Family 2. Obolidae. King. 



Georgia, Vermont. A, 



Thick-shelled Obolacea, of nearly circular or ovoid outline, yjjjjjj \^^ $ l \ ou ' 

 biconvex, usually smooth, and with rudimentary cardinal areas specimen, s/j "(after 

 traversed by shallow pedicle grooves. Muscular scars distinct, 

 consisting of two pairs of adductors and three of sliders, or adjusters. Cambrian. 



Under this family are included some of the oldest known Brachiopods. The 

 family is abundantly represented in the Lower and Middle Cambrian. 



Obolella, Billings (? Dicellomus, Hall). Small, oval, or round shells, with 



Bill. 



FIG. 505. 



1'uterina Labr<" 



Cambrian ; near 



FIG. 506. 



Obolus Appollnis, Eichw. Cambrian ; St. Petersburg, Russia. A, Exterior 

 of dorsal valve. , C, Interior of ventral valve. D, Interior of dorsal valve 

 Natural size. 



Fio. 507. 



Spondylobolv* 

 luris, M'Coy. Ordovician ; 



Ireland. Natural si/r 

 (after Davidson). 



narrow cardinal' areas. Dorsal valve usually with a rudimentary hinge facet. 

 North America and Europe. 



VOL. I Y 



