BRACHIOPODA. 
13 
Fam. 1. TEREBRATULIDiE. 
Shell minutely punctate, usually round or oval, and smooth 
or striated ; ventral valve (fig. 2) largest, its umbo produced into 
a beak with the apex truncated and perforated ; foramen [f) sepa- 
rated from the hinge-line by a small triangular plate or deltidium 
(d) composed of two pieces (deltidia ) ; teeth {t) situated one on 
each side of the deltidium, supported by plates ; dorsal valve 
(fig. 1) with a depressed umbo; furnished interiorly with a pro- 
minent cardinal process (j) between the sockets for the teeth (t’), 
a hinge-plate (p) with four cavities and a central ridge or septum 
(s) ; internal skeleton in the form of a slender shelly loop, at- 
tached by its crura (c) to the hinge-plate, and furnished near its 
origin with oral processes (c). 
Fig. 2. 
Fig.l. 
Fig. 1. Dorsal valve cardinal process; dental sockets ; p, hinge-plate; 
s. septum ; c. crura of the loop ; i. reflected portion of the loop ; m. quadruple 
adductor-impression . 
Fig. 2. Ventral valve:—/, foramen; d. deltidium; t, teeth; n. adductor-im- 
pression; r. retractor impressions ; pedicle-muscles; anal muscles; v. posi- 
tion of the vent ; z. attachments of peduncle-sheath. 
Animal always attached by a peduncle ; furnished with cirrated 
arms, united throughout by membrane, folded upon themselves, 
and only spiral at their extremities ; muscles of three kinds, — 
adductors, retractors, and those which go to the peduncle [byssal 
or pedal muscles of ordinary bivalves). The adductors are at- 
tached to four spots near the centre of the dorsal valve {m), and 
to a single spot behind the centre of the opposite valve {a ) ; the 
retractors originate on each side of the adductor in the ventral 
valve (r), and are attached to the cardinal process (j) of the dorsal 
valve ; the hinge-teeth form the fulcrum on which the dorsal 
valve turns ; of the pedicle muscles, two (p) originate outside the 
adductor and behind the retractors in the ventral valve ; two 
