FAMILY 10. ARCACEA. 109 



The genus Nucula presents a most anomalous union of characters, 

 and no little embarrassment arises in deciding its natural situation in 

 the system. In following the arrangement of Lamarck, we are guided 

 by the anatomical observations of M. Quoy, who clearly shows that the 

 animal of Nucula has a decided affinity with that of the Arcacea ; but in 

 addition to this important testimony, it will be found that Gray still 

 includes this genus with his family of the Arcades. We find, nevertheless, 

 upon examining the shell, that a strong opposing character exists in the 

 position of the ligament, it being internal, inserted in a small central car- 

 dinal pit within each valve. For this reason the genus in question has 

 been naturally referred by several eminent conchologists, Turton, Leach, 

 Sowerby, &c. to the family of the Mactracea. But since these authors 

 wrote upon the subject, the discovery of a more extended series of 

 Nucula? has enabled us to lay out two separate and distinct divisions of 

 species : in one we observe that the shell is thin, and covered with a green 

 horny epidermis ; in the other it is thick and solid, with a dark brown 

 epidermis ; the interior of the latter differs also from that of the former 

 in being pearly. Now the principal generic characters of each are the 

 same ; yet if the position of the ligament were not regarded, the former 

 division of species might with just as much propriety be placed near 

 Solenella in the family of the Solenacea. By the internal ligament the 

 Nucula? belong to the Mactracea : this, however, is the only character 

 which they exhibit in common with that family ; we must be therefore 

 guided by the affinities of the animal, which, as before stated, have been 

 satisfactorily decided as approaching to the Arcacea. 



The shell of NucuJa may be described as being transverse, either thin 

 or solid, equivalve, inequilateral, often pearly within, and covered exter- 

 nally with either a green, or dark olive-brown, epidermis ; theumbones are 

 contiguous, there being no intermediate area as in the rest of the Arcacea 

 in consequence of the change in the position of the ligament. The hinge 

 is linear, with an internal obliquely-produced pit in the centre for the 

 reception of the ligament ; the teeth are numerous, sharp, rather recurved, 

 set in a straight series on each side of the ligamentary pit. The muscu- 

 lar impression of the mantle is entire. 



