NATURAL HISTORY. 
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have a very small anterior tooth and concentrically ridged shells ; 
the rest of the genera have no anterior teeth. The Mercenaries have 
an angular, and the Anomalocardia no syphonal impressions. The 
Tapes and Venerupes have oblong shells with very compressed teeth, 
and the Clementia are like the latter, but are very thin, and have a cavity 
in the margin before and behind the teeth. 
The family of Cyrenhle are very like, and have all the charac¬ 
ters of the former, but they are fluviatile, living in rivers and streams, 
and the shells have distinct lateral teeth, and are covered with a hard 
olive polished periostraca. The Cyrence have three teeth in each valve, 
and the compressed lateral teeth striated across. The Geloina differ in 
the lateral teeth being smooth ; and the Velorita has a short thick 
anterior lateral tooth close to the large cardinal ones; they are con¬ 
fined to warm climates. The Cyclades are small shells found in colder 
climates, with two teeth in one valve and three in the other. The 
Pisidiums differ from the latter in being oblong, with the beak nearer 
the front extremity; the former have two separate and the latter only 
one syphon. They are viviparous. 
The family of Cockles (Cardiad^e) have very short syphons, not 
leaving any syphonal inflections on the shell. Their foot is large 
and sickle-shaped, and the shell cordate and radiately ribbed, with two 
large teeth in each valve, placed in a cross, and distinct lateral teeth. 
The ligament is external. The Cardiums are heart-shaped. The 
Hemicardiums are very short and strongly keeled, so as to be several times 
broader than long; the hinder side is sometimes so much compressed as 
to be mistaken for the anterior one. The Conocardia , which are only 
known in the fossil state, are like the latter in shape, but the keel is 
furnished with a large expanded edge, and the cardinal line is extended 
into a point at each end. 
The family of Mactrid^: have the cartilage separated from the 
ligament, and placed in a triangular pit behind the cardinal teeth. 
Their anterior hinge tooth is bifid, or triangular. In Scissodesma the 
ligament is placed in an external slit just over the cartilage; in Mactra 
it is external in a marginal groove; and in Spisula and Lutraria it is 
sub-external, marginal on the upper part of the cartilage; in the former 
the hinder lateral teeth are double and single, and in the latter single or 
wanting. In Mulinia and Gnathodon, the ligament is internal, and 
placed in the same pit as the cartilage; in the former the lateral teeth 
are simple like Mactra, and in the latter the front lateral tooth is hatchet - 
shaped, and the shell wedged-shaped. 
The Mesodesmule only differ from the Mactridce in the anterior 
cardinal tooth being single and compressed. The Mesodesma are ovate 
shells with the hinge in the middle; the Donacillce are wedge-shaped 
shells with the hinge at one end; both have distinct syphonal inflections. 
The Anopa are triangular and have^, syphonal inflections. 
The family of Tellinid^: have very long slender syphons, which 
are separated from each other in their whole length ; their shells 
are consequently marked with a very large syphonal inflection. The 
cartilage is generally external. The Psammobia and Psammotea have 
elongate shells, rather gaping at each end. The Tellince have a fold 
on the hinder slope. The Arcopagioe differ from the latter in being 
rounder and more solid, and the Strigellee have the shell obliquely 
