nRACITIOl'ODA. 



i;< 



Fam. 1. TKREBRATULID.'E. 



Shell minutely punctate, usually round or oval, and smooth 

 or striated ; ventral valve (tig. 2] lar<^est, its uinlto produced into 

 a l)eak witii the apex truncated and perforated ; Inranieu (/) sepa- 

 rated from the hinge-line by a small triangular plate or ilellidiuin 

 (cl) composed of two pieces {dellidui) ; teetii (/) situated one on 

 each side of the deltidium, supi)orted by plates ; dorsal valve 

 (fig. 1 ) with a depressed umbo ; furnished interiorly with a pro- 

 minent cardinal process (j) between tlie sockets for the teeth (/'), 

 a hinge-jjlate {p) with four cavities and a central ridge or septum 

 (s) ; intermil skeleton in the form of a slender shelly loop, at- 

 tached l)v its crura (c) to the hinge-plate, and furnished near its 

 origin with oral processes (c). 



Fig. 2. 



Fig. 1. Dorsal valve ;— J. cardinal process; t'. dental sockets ; p. hinge-plate; 

 s. septum; r. crura of the loop; /. reflected portion of the loop; m. quadruple 

 adductor- impression . 



Fig. 2. Ventral valve:—/, foramen; d. deltidium; t, teeth; a. adductor-im- 

 pression ; r. retractor impressions ; /;. pedicle-muscles ; .i . anal muscles ; v. posi- 

 tion of the vent ; z. attachments of peduncle-sheath. 



/I «jm«/ always attached by a peduncle; furnished with ciiTated 

 arms, united throughout by membrane, folded u|)on themselves, 

 !uul only spiral at their extremities ; muscles of three kinds, — 

 adductors, retractors, and those which go to the peduncle {byssal 

 or pedal muscles of ordinai-y bivalves). The adductors are at- 

 tached to fom* 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 



