NUCULA. 141 



cartilage ; while Limopsis has the form of Pectunculus, 

 with a cartilage and a cavity for its reception, as in the 

 so-called Nuculida, although placed differently, viz. out- 

 side the hinge, instead of in the middle of it. Stimpson 

 advocates the separation of Nucula from the Arcidcs be- 

 cause of the want of a byssus ; but Pectunculus has none. 

 This appendage is only necessary when the habitation is 

 rocky or " hard/' In sandy and muddy, or " soft " 

 ground the foot is used for burrowing, instead of spin- 

 ning a byssus; and it is consequently larger in Nucula, 

 Leda, Limopsis, and Pectunculus, which live in such 

 situations, than in Area, which attaches itself to rocks 

 and old shells. Recluz suggested the removal of Leda 

 from the Arcidcs and Nuculidae, and considered that all 

 of them belonged to different tribes ; but our progress in 

 classification has not yet advanced sufficiently to allow 

 of such extreme subdivision. Gray placed Nuculida 

 between the families represented by Solen and My a, and 

 in another order than that which contains Area, Pec- 

 tunculus and Limopsis. Leach's arrangement is rather 

 more eccentric or opposed to general views. His fami- 

 lies of Nuculada and Pectunculidae lie between Mactra 

 and Venus, and his Arcades (longo intervallo) between 

 Pinna and Avicula. The late Mr. G. B. Sower by was 

 the first to point out the separation of Leda and Nucula 

 from Area. 



A. Inner margin notched. 



( &Tek-) r bl 7-0 . 



,V*V7 1- NUCULA SULCA TA *, Bronn. 



N. sulcata, Bronn, Italiens Tertiargebilde, p. 109, no. 633. N. decussata, 

 F. & H. ii. p. 221, pi. xlvii. f. 1-3. 



SHELL obtusely triangular, rather convex, solid, of a dull 

 hue: sculpture, numerous transverse striae or ridges, which 



* Furrowed. 



