ACEPHALA TESTACEA. 281 



tion parallel to the hinge, than the Cardia. Therib«, when there are any, 

 are almost always parallel to the edges, being directly the reverse of their 

 arrangement in the Cardia. 



The animal is always furnished with two more or less protractile tubes, 

 sometimes united, and with a compressed foot, which enable it to crawl. 

 This genus is now divided into Petricola, Crassina, Cytherea, Capsa, Cor- 

 bula, Madra, &c. The common Clam is a true Venus. 



FAMILY V. 



INCLUSA. 



The mantle open at the anterior extremity, or near the middle 

 only, for the passage of the foot, and extended from the other end 

 into a double tube, which projects from the shell, whose extremities 

 are always gaping. Nearly all of them live buried in sand, stones, 

 ooze or wood. Those of the genus 



Mya, Lin., 

 Have but two valves to their oblong shell, the hinge of which varies. The 

 double tube forms a fleshy cylinder, and the foot is compressed. The 

 different forms of the hinge have furnished Messrs Daudin, Lamarck, &c. 

 with various subdivisions, Luiraria, Myaproper, Anatina, Solemya, &c. 



Here also we find a group of some small and singular genera, 

 such as 



Btssomia, Cuv. 

 Where the oblong shell, which has no marked tooth, has the opening for 

 the foot at about the middle of its edge and opposite the summits. The 

 Byssomix penetrate into stone, oorals, &c. 

 A species which is provided with a byssus, abounds in the Arctic Ocean. 



HiATELLA, Daud. 

 The shell gaping, to allow the passage of the foot, near the middle of its 

 edges; but the tooth of the hinge is better marked than in the preceding 

 genus. Ranges of salient spines are frequently observed on the hind part 

 of the shell. They are found in sand, among Zoophytes, &c. 



SoLEN, Lin. 



The shell only bivalve, oblong or elongated, but the hinge always furnished 



with salient and well marked teeth, and the ligament external. In the 



Solens properly so called, the shell is cylindrically elongated, and has two or 



2 L 



