20S MALACOZOA. TROFIOPODA. LAMELLIBRANCHIATA. 



short, thick, truncated ; two adductor muscles. Shell 

 equivalve, inequilateral, with an epidermis ; the hinge 

 with numerous small interlocking teeth ; the ligament 

 partly external. 



Genus 1. Nucula. — Shell ovato-trigonal or oblong, in- 

 equilateral ; umbones small, contiguous ; incurved ; 

 hinge with a central oblique cavity, and on each side 

 numerous small, compressed teeth. Nucula, diminutive 

 of Nux, a nut. Lamarck. 



1 . Nucula Nucleus. — Shell obliquely ovato-triangular, 

 with the margin internally crenate. Nucleus, a nut. 



2. Nucula tenuis. — Shell obliquely ovate, with the 

 margin internally smooth. Tenuis, thin. 



3. Nucula rostrata. — Shell elongated, with the pos- 

 terior end rounded, the anterior extended, narrowed, 

 and truncate ; the surface concentrically rugoso-striate. 

 Rostra tus, having a beak. 



4. Nucula minuta. — Shell ovato-lanceolate, with the 

 posterior end rounded, the anterior extended, narrowed, 

 and truncate ; a depression under each umbo, separating 

 the teeth ; the surface concentrically striulate, with a 

 few convex ridges toward the margins. Minutus, very 

 small. 



Family V. — Cycladina. 



Animal oval or roundish, more or less compressed, 

 with the mantle-lobes free beneath, united behind, and 

 forming an internally double tube ; the foot very ex- 

 tensile, elongated. Shell suboval or roundish, very thin, 

 with a delicate epidermis ; the umbones prominent ; the 

 hinge with two or three divergent laminar teeth ; the 

 ligament external. Name from the Genus Cyclas. 



Genus 1. Cyclas. — Shell elliptical or oval, subglobose, 

 very thin ; umbones tumid, obtuse, about the middle ; 

 siphons separated toward the end. Named from its 

 globular form ; Cyclus, a circle. Lamarck. 



1. Cyclas Jlavescens. — Shell rhomb oido- elliptical, ven- 

 tricose, somewhat inequilateral; the valves very thin, 



