CLASSIFICATION CAMBRIAX BRACHIOPODA — WALCOTT I57 



Heart-shaped Cavity. — Central depressed portion of visceral 

 area (Mickwitz). 



Hinge Line. — The line along which articulation takes place ; also 

 sometimes developed among inarticulate brachiopoda. 



Inarticulate Brachiopoda. — In the orders Atremata and Neo- 

 tremata the valves do not, as a rule, articulate by means of teeth 

 and sockets, as is the case in the articulate orders Protremata and 

 Telotremata. 



Lateral Areas. — That portion of the shell on each side of the 

 longitudinal axis. 



LiSTRiuM. — In some Neotremata a plate closing the progressive 

 track of the pedicle opening or pedicle cleft, posterior to the apex of 

 the ventral valve. 



Longitudinal Axis. — A median line through the shell from the 

 beak to the anterior margin. 



Median Septum. — An internal vertical plate commonly developed 

 along the longitudinal axis and between the muscles of the ventral 

 valve. Sometimes there is also a dorsal median septum. Lateral 

 septa are rarely developed. 



Middle Lateral Muscle Scar. — See Outside lateral. 



Neanic. — Designating youthfulness, or the stage in which specific 

 characters begin to develop. 



Neotremata. — Circular or oval, more or less cone-shaped, inar- 

 ticulate calcareo-phosphatic brachiopods with the pedicle opening 

 restricted throughout life to the ventral valve. (For a more de- 

 tailed description see page 145.) 



Nepionic. — Designating the smooth shell stage succeeding the 

 protegulum. 



Outside and Middle Lateral (Protractor) Muscles. — In the 

 Obolidas one pair has the ventral ends fastened at the anterior ex- 

 tremity of the visceral area, extending backward and inserted near 

 the lateral margin of the dorsal valve, outside the transmedians. A 

 second pair originates just behind the centrals of the ventral valve 

 and is inserted posterior to the first pair. These muscles draw the 

 dorsal valve forward. 



Parietal Band. — The point of attachment of the muscular wall 

 surrounding the visceral area. 



Pedicle. — The flexible muscular organ of the ventral valve by 

 means of which brachiopods may be attached to extraneous objects. 



Pedicle Furrow. — The external furrow adjoining the foramen 

 •or pedicle opening in certain Neotrematous genera. 



