igo 



AN INTRODUCTION TO THE STUDY OF FOSSILS 



3. Higher cardinal area (vertical distance from hinge line 

 to beak) than in the brachial valve. 



4. Beak incurved over that of brachial valve. 



Brachiopod shells often appear to be very similar to pelecy- 

 pod shells, but may usually be readily distinguished from them 

 by one or more of the following characters : 



I. 



2. 



3- 



Brachlopoda 

 Equilateral 

 Inequivalved 



Pedicle opening present (ex- 

 cept in Atremata) 



4. Teeth in one valve, sockets 



in the opposite valve 

 (except in Inarticulata) 



5. No ligament present ; 



valves opened by muscles 



6. Valves dorsal and ventral 



Pelecypoda 



1. Inequilateral 



2. Equivalved (generally) 



3. No pedicle opening present 



(in some a byssal notch 

 or hole) 



4. Teeth and sockets in each 



valve (typically) 



5. Valves not opened by mus- 



cles but by ligament or 

 resilium at hinge line 



6. Valves right and left 



It is suggestive of retrogression in the Articulata sub-class 

 that while in living species the anus ends blindly, in Paleozoic 

 forms it was probably functional. In Rensselaeria, Athyris, 

 Atrypa, Rhynchonella, etc., the beak of the brachial valve is 

 notched or perforate (between floor of valve and union of the 

 bases of the arm supports) for its passage to the exterior. In 

 Strophomena and Stropheodonta it passed between the branches 

 of the cardinal process. In living Inarticulata, as Lingula, the 

 digestive canal ends in an open anus. 



All brachiopods are in the larval stage free-swimming, 

 sometimes for a very short time (longest among Inarticulata) ; 

 it is hence during this period that their distribution in space 

 takes place. The cause of the distribution is mainly one of 

 ocean currents, since the microscopic larva, about a third of a 

 millimeter long, swims about by a twirling motion for compara- 

 tively few hours and at a rate which could carry it only about 

 a yard an hour. 



