ORDER I ATREMATA 305 
of growth, the austral group (Magellaninae) passes through a different series 
of loop metamorphoses from the boreal one (Dallinae). 
It was by the application of the above-mentioned principles that Schuchert, 
in 1893, arranged all the genera of Brachiopoda under the four orders 
instituted by Beecher. Further attention has since been given to this subject 
by the same writer, and the classification outlined in his Synopsis of American 
Fossil Brachiopoda has been adopted in the present work. 
Order 1. ATREMATA. Beecher. 
Inarticulate Brachiopoda with the pedicle emerging freely between the two valves, 
the opening being more or less shared by both. Growth taking place mainly around 
the anterior and lateral margins, never enclosing or surrounding the pedicle. Aperture 
unmodified. Prodeltidiwm 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, 
and 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 protequlum, or initial shell, or obably 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. 

Fic. 505. 
: ; = Paterina Labradorica, 
Family 2, Obolidae. King. Bill. Cambrian; near 
Georgia, Vermont. 4, 
Thick-shelled Obolacea, of nearly circular or ovoud outline, el sare: aoe 
biconver, usually smooth, and with rudimentary cardinal areas specimen, % (after 
traversed by shallow pedicle grooves. Muscular scars distinct, Foote 
consisting of two pairs of adductors and three of sliders, or adjustors. 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 (? co Hall). Small, oval, or round shells, with 

Fic. 507. 
Fia. 506. G ; . 
es ; Spondylobolus  cranio- 
Obolus Appolinis, Eichw. _Cambrian ; St. Petersburg, Russia. A, Exterior laris, M‘Coy. Ordovician; 
of dorsal valve. B, C, Interior of ventral valve. JD, Interior of dorsal valve. Ireland. Natural size 
Natural size, (after Davidson). 
narrow cardinal areas. Dorsal valve usually with a rudimentary hinge facet. 
North America and Europe. 
VOL. I x 
