MYACEA. 741 



Trias, and the genus still survives. The Carboniferous genus 

 So/enopsis, with prominent beaks and a closed anterior end, and 

 the Devonian Palceosolen may represent ancient types of the Solen- 

 idte. Here also, perhaps, may be placed the Palaeozoic genus 

 Orthonota, the typical forms of which are Devonian, though species 

 have been described from rocks as 

 old as the Ordovician (fig. 624). In 

 this genus the shell is greatly elongat- 

 ed and very inequilateral, the anterior 

 end being rounded and the posterior 

 end truncate, and the beaks being 

 close to the anterior end. The 

 muscular impressions and pallial line 

 have not been made out; and the Y\%.e^.—Orthonota{prthodesmd)par- 

 hinge is only imperfectly known, but allela - ° rdovician - 



is apparently edentulous. The hinge-line is straight, and in the 

 typical forms of the genus is continued in a straight line in front of 

 the beaks. In one group of forms, however, sometimes distinguished 

 under the name of Orthodesma, the hinge-line is bent or contracted 

 in front of the beaks, and is straight behind (fig. 624). 



Order X. Myacea. 



In this order the mantle-lobes are united, a pedal aperture exist- 

 ing in front and two siphons behind. The siphons are long, and 

 may be united or separate. The foot may or may not be byssiferous, 

 and there are two adductor muscles. The shell is free, equivalve 

 or inequivalve, with an internal or external ligament, and a variable 

 pallial line, while the characters of the hinge also differ in different 

 families of the order. The chief families of this order are the 

 Mactridce, Myidcz, Glycimeridce, and Gastrochcenidce. 



Family i. Mactrid^e. — In this family the mantle is more or 

 less open in front ; the siphons are united, with fringed orifices ; 

 and the foot is compressed. The shell is equivalve, trigonal, gen- 

 erally gaping behind ; the ligament being typically internal and 

 lodged in a deep triangular pit between the beaks. The left valve 

 has a widely bifurcated cardinal tooth, which is received between 

 the diverging branches of a right cardinal tooth ; and lateral teeth 

 are usually present in addition in front and behind. The pallial 

 line is usually deeply sinuated. The members of this family are 

 all marine, and the two principal genera are Mactra and Lutraria. 



In the genus Mactra (fig. 625) the shell is trigonal in form, 

 slightly gaping behind, with a short pallial sinus. The internal 

 ligament is contained in a triangular pit, but there is also an 

 external ligament lodged in a groove. The recent species of 



