19-2 



the ligament external, tortuous, and partly penetrating into 

 the shell. 



A very rare large shell, of beautiful pearly lustre, ad- 

 herent to the rocks at great depths. Only known in a 

 recent state.* 



TridaciKE. — Transverse, equivalved ; one large laterally 

 extended impression. 



1. Tridacna. — A regular, equivalved, inequilateral, 

 transverse bivalve, gaping at the lunule ; the hinge with two 

 compressed, unequal, forward, inserted teeth ; the ligament 

 external and marginal. Recent and fossil ? — PI. vii. fig. 26. 



2. Hippopus. — Differs from the preceding in its luntile 

 being close. 



MytilacecB. — Hinge, an almost internal, marginaj, linear 

 ligament. The inhabitants of many of these shells attach 

 themselves by a byssus. 



1. Modiola. — A subtransverse, equivalved bivalve, very 

 short on the hinder side ; the beaks sublateral, lying on the 

 shorter side ; the hinge lateral, linear, and without teeth ; 

 the cardinal ligament rather internal, placed in the marginal 

 groove ; one sublateral, hatchet-formed impression. Recent 

 and fossil. — PI. vi. fig. 25. 



2. Mytilus. — A longitudinal, equivalved, regular bivalve, 

 pointed at its base ; the beaks nearly straight, terminal and 

 pointed ; the hinge lateral, generally without teeth ; the 

 ligament marginal, and rather internal ; one long, clavated, 

 and sublateral impression. Recent and fossil. — PI. vi. fig. 24. 



3. Pinna. — A longitudinal, wedge-formed, equivalved 

 bivalve, gaping at its summit, pointed at its base, and having 



* This genus is beautifully illustrated in Mr. G. Sowerby's 

 work on the genera of shells, from which this definition has been 

 taken. 



