236 MOLLUSCA. 



Allied to Cardita is the extinct genus Hippopodium, well 

 known by the thick and solid H. ponderosum of the Lias. 



Pachyrisma is another extinct genus, in which the shell is 

 also very thick and ponderous in its structure. It has large 

 sub-spiral umbones, and is peculiar to the Great Oolite. 



Megalodon is likewise extinct, and includes massive shells, 

 with sub-spiral beaks and an external ligament. The genus 

 is doubtfully represented in the Silurian and Carboniferous 

 Rocks, and is characteristically Devonian. 



Lastly may be mentioned the shells which are known as 

 Anthracosia, which abound in parts of the Carboniferous 

 series. These are nearly allied to the extinct genus Cardinia, 

 if they do not actually belong to it. They are exclusively 

 Palaeozoic, and extend from the Upper Silurian to the 

 Carboniferous ; whereas Cardinia is very doubtfully repre- 

 sented in rocks older than the Lias. 



Sub-division II. Sinupallialia. Respiratory siphons large; 

 pallial line indented. 



FAM. 14. VENERID^E. Shell regular, sub-orbicular or oblong; 

 ligament external ; hinge with usually three diverging teeth in 

 each valve. Animal usually free and locomotive ; mantle with 

 a rather large anterior opening ; siphons unequal, more or less 

 united. Foot tongue-shaped, compressed, sometimes grooved 

 and byssiferous. The Vaieridce are the most highly organised 

 of the Bivalves, and comprise some of the most beautiful 

 examples of the class. They commence in the Oolitic Rocks, 

 are abundant in the Tertiaries, and have attained their maxi- 

 mum at the present day. All the more important fossil forms 

 belong to the nearly-allied genera Venus and Cytherca. Both 

 of these commenced their existence in the Oolites, the former 

 being represented by about one hundred and fifty, the latter 

 by nearly a hundred extinct forms. 



FAM. 15. MACTRID^E. Shell equivalve, trigonal; hinge with 

 two diverging cardinal teeth, and usually with anterior and pos- 

 terior lateral teeth. Mantle more or less open in front ; siphons 

 united, with fringed orifices ; foot compressed. The only two 

 genera of any importance as fossils are Mactra and Littraria, 

 both of which live buried in sand or mud. The Mactra have 

 a nearly equilateral shell, with a short pallial sinus, and an 

 internal ligament contained in a triangular pit. They appear to 

 have commenced in the Lias, and have attained their maximum 

 at the present day. In Lutraria the shell is oblong and gaping 

 at both ends, the pallial sinus is deep, and the internal ligament 

 is supported by a prominent cartilage-plate. The genus is not 

 known in rocks earlier than the Miocene Tertiary. 



