80 PALAEONTOLOGY 



IV. HETERODONTA. 



Equi- or inequi-valve ; isomyarian ; integri- or sinu- 

 palliate ; internally porcellanous. Hinge-lamellae differ- 

 entiated into anterior-lateral and cardinal teeth, with 

 frequent development of posterior-laterals behind the 

 shortened ligament. Ligament opisthodetic, usually 

 external, sometimes internal (resilium). Ornamentation 

 varied. Jurassic to Recent. 



This very large and important order falls into two 

 series, in each of which a number of sub-orders may be 

 recognized. 



A. WITH TEETH OF LUCINOID TYPE : 3^_f, 



2, 46 



1. Cardiacea. With radial ornament, at first (Proto- 

 cavdia) in siphonal region only, later on whole surface. 

 Umbones fairly central. Anterior and posterior laterals 

 well developed. 



The common cockle, Cardium (Fig. 23, g), has a rounded 

 outline, a central umbo, with well-defined and separated 

 anterior-lateral, cardinal and posterior-lateral teeth ; the 

 whole surface is ornamented by strong radial costae. In 

 the Mesozoic Protocavdia radial ornament is confined to the 

 posterior end (i.e., to the part secreted by the siphonal 

 part of the mantle) the rest being concentrically marked. 

 (Compare the case of Trigonia.) In the Aralo-Caspian 

 region, from Miocene to Recent, are found Adacna and 

 Limnocardium, cockles adapted to a freshwater or brackish 

 habitat, with long siphons and a pallial sinus. Doubtfully 

 referred to this sub-order is Thetironia, common in the 

 Lower Greensand of the Isle of Wight, with a myophoric 

 ridge that has been mistaken for a peculiar pallial sinus. 



2. Rudistes. Highly aberrant fixed forms, probably 

 derived from Cardiacea. Inequivalve. Fixed either by 

 left valve (normal) or right (inverse) ; both valves spiral 



