MALACOXOA. TROPIOFODA. LAMKLLIBRANCHIATA. 243 



Test Brit. 33. Alasmodon margaritiferum. Flem. Brit. Anim. 

 417. Alasmodon margaritiferus. Gray's Turt. Man. 293. Mya 

 margaritifera. Penn. Brit. Zool. iv. 80. PI. 43. f. 18. Unio mar- 

 garitiferus. Turt. Brit. Biv. 242. PI. 16. f. 1. 



FAMILY IV. ARCINA. 



Animal oblong, or roundish ; with the mantle open 

 in its whole length, unless on the back, and destitute of 

 tubes ; a very short, thick, truncated foot ; two adduc- 

 tor muscles, of which the posterior is larger. 



Shell regular, equivalve, inequilateral, covered with 

 an epidermis ; the umbones generally widely separated ; 

 the hinge with numerous small interlocking teeth, dis- 

 posed in an elongated straight or curved series ; the liga- 

 ment partly external; the muscular impressions sub- 

 marginal, the anterior smaller. 



GENUS 1. NUCULA. NUCULE. 



Animal roundish, subtriangular, or oblong, com- 

 pressed. The lobes of the mantle disunited. The 

 mouth with two pairs of very narrow and elongated 

 palpi. Foot compressed, slit at the end. Abdominal 

 mass rather thin, with the foot attached to it in its 

 whole length ; on each side a pair of elongated, very 

 narrow, filamentous branchiae. Anterior adductor 

 muscle small, posterior moderate. 



Shell oval, ovato- trigonal, or oblong, equivalve, 

 inequilateral, concentrically striated. Umbones small, 

 contiguous, incurved. Hinge with a central oblique 

 cavity, and on each side a series of small, compressed, 

 projecting teeth. Ligament chiefly internal, in the 

 cavity of the hinge. 



1. Nucula Nucleus. Common or Pearly Nucule. 



Shell obliquely ovato-triangular, moderately convex, smooth- 

 ish, obsoletely striated transversely, with smaller and more 

 obscure longitudinal striae ; the surface whitish, with trans- 

 verse irregular reddish markings, the epidermis olivaceous : 

 the umbones prominent, the anterior slope abruptly descend- 

 ing, with an elongated impression, the dorsal slope descending 



