268 MOLLUSCA. 



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 li ve buried in sand, stones, ooze or wood. Those of the genus 



MYA, Linnaeus, 



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, Lutraria, Mya proper, Anatina, Solemya, &c. 



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



BISSOMIA, Cuvier, 



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 Byssomia? 

 penetrate into stone, corals, &c. 

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



HIATELLA, Daudin. 



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, Linnaeus. 



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 three 

 teeth in each valve near the anterior extremity, where the foot issues. The 

 latter is conical, and enables the animal to bury itself in the sand, which it 

 excavates with considerable rapidity on the approach of danger. 



PHOLAS, Linnaeus. 



The Pholades have two broad valves, convex towards the mouth, narrow 

 and elongated on the opposite side, and leaving a large oblique opening at each 

 extremity ; their hinge, like that of a true Mya, is furnished with a plate pro- 

 jecting from one valve into the other, and with an internal ligament running 

 from that plate into a corresponding cavity. Their mantle is reflected exter- 

 nally upon the hinge, where it sometimes contains two or three supernumerary 



