488 



LAMELLIBRANCHIATA. 



ventricose and very inequilateral, the umbones being anterior 

 (fig. 349). The hinge -area is undefined, and the surface 

 generally smooth. There are a few (three) anterior cardinal 

 teeth, and " two or three remote oblique posterior teeth par- 

 allel to the hinge-margin" (Salter). The species of Cyrto- 

 donta appear to be exclusively confined to the Silurian and 

 Devonian rocks. Vanuxemia comprises forms of the genus 

 with nearly terminal beaks. In the genus Megalomus, again 

 (fig. 350), are comprised forms essentially similar to Cyrto- 

 donta (possibly identical with it), but having an excessively 

 thickened and massive shell, which is usually strongly in- 

 flated. The valves are equal ; the beaks strongly incurved 



Fig. 350. Megalomus compressus. A, Side view of the cast of the shell, natural size ; B, The 

 same viewed from above, ad, Cast of the adductor impression ; p, Pallial line. 



and placed anteriorly; the hinge-line furnished with three 

 or four (?) strong transverse teeth; and the anterior muscular 

 impression extremely deep. The forms of this genus appear 

 to be confined to the Upper Silurian, and often attain a great 

 size, but they usually occur in the state of casts only. 



Coming next to the grojip of Arks included under the 

 name of Nuculidce, we find that Nucida itself (fig. 351) has 

 a trigonal shell, the beaks of which are reversed, and turned 

 towards the posterior side of the shell, which is also the 



