THE LAMELLIBRANCHIA 77 



share with the brachiopod Lingula the honour of being 

 among the longest-lived genera of animals. 



Ctenodonta has some resemblance in form to Pectunculus, 

 but is far earlier in time (Ord.-Sil.). 



Probably allied to the Nuculacea are two Silurian- 

 Devonian groups (i) Actinodonta (Fig. 23, b), Lyrodesma, 

 etc., which show a change of the hinge-teeth from the 

 parallel series of Nucula to a radiating series; and (2) 

 Cardiola and Buchiola, with strong radial ornament and 

 apparently toothless hinge. 



II. NAIADACEA. 



Equi valve, strongly inequilateral ; isomyarian ; inte- 

 gripalliate; internally nacreous. Ligament external, 

 opisthodetic. Hinge-teeth lamellar. Rarely ornamented 

 otherwise than by concentric striae. Freshwater habitat. 

 Devonian- Recent. 



The "freshwater mussels "are probably derived from 

 forms like Actinodonta by the restriction of the ligament, 

 various forms from the Permian of Russia showing 

 hinge-teeth of taxodont to radial type. On the other 

 hand, as early as the Devonian, there is found an appar- 

 ently toothless form, Archanodon ; and the hinge-structure 

 of the common Carbonicola of the Coal Measures (Fig. 23, c) 

 is imperfectly known. From Mesozoic time down to the 

 present day the genus Unio shows a hinge which is 

 transitional between the actinodont and heterodont types : 



its formula may be given as -- -^ (3a, 4a, and 5 



20, o, 40, * 



are usually described as "anterior-laterals," 20, as a 

 " cardinal,"and the other three as " posterior-laterals " 

 an incorrect term, since they extend below the ligament). 



III. PR^-HETERODONTA. 



Equivalve, inequilateral, isomyarian, integripalliate ; 

 internally nacreous or porcellanous. Hinge-teeth con- 



