NUCITLA. 217 



nearly straight, with one narrow curved cardinal tooth in one 

 valve, entering between two similar teeth in the other ; siphonal 

 scar long; extremities of the shell gaping ; epidermis thin, light 

 brown, folding over the edges and connecting the dorsal margins; 

 Hab. Jumna, Grooti, and Ganges. PI. iii. fig. 63, 



NUCINELLA. S. Wood. Biv. Crag. 1850. A genus formed 

 for the reception of a very minute crag fossil belonging to the 

 same family as the Nucules and PectunctJi ; but difiering in the 

 arrangement of the teeth. N. miliaris. PL xxvii. fig. 579. 



NUCLEOBEANCHIATA. Bl. The fifth order of the second 

 section of Paraeephalophora Monoica, Bl. the shells of which 

 are described as symmetrical, more or less curved, or longi- 

 tudinally rolled up and very thiu. This order contains, Fam. 1. 

 Nectopoda, containing Carinaria ; Fam, 2. Pteropoda, contain- 

 ing Atlanta, Spiratella and Argonauta. 



NUCLEUS. {A kernel^ Anything forming a centre around 

 which matter is gathered. The nucleus of shells is the first 

 formed part ; the first deposit of shelly matter to which the 

 successive layers are added; -the apex of the spiral cone, of 

 which most shells are composed. (See Cone.) The nucleus is 

 formed within the egg in oviparous, and within the old shell in 

 viviparous moUusca. It is frequently more transparent and 

 light than the remainder of the shell, and sometimes falls off ; 

 when this occurs the shell is said to be decollated. 



NUCULA. Lamarck, 1801. {A small nut.) Fam, Arcacea, Bl. 

 and Lam. — Bescr. Equivalve, inequilateral, transverse, covered 

 with an epidermis ; hinge linear, with a series of sharp, angu- 

 lated teeth", arranged in a line on each side of the umbones, and 

 central ligamentary pit ; muscular impressions two, simple . 

 palleal impressions not sinuated.— Ofis. The row of teeth on 

 each side of the umbones, and the ligamentary pit in the centre 

 of the hinge prevent the pretty little shells of this genus jfrom 

 being confoimded with any other. Thirty-four figures are enume- 

 rated in the catalogue by Sowerby, sen. which accompanies the 

 Conchological Illustrations of the author. The new species, to 



