204 NUMMULACEA. 



NUCLEOBRANCHIATA. Bl. The fifth order of the second section 

 of Paracephalophora Monoica, Bl. the shells of which are described 

 as symmetrical, more or less curved, or longitudinally rolled up 

 and very thin. This order contains, Fam. 1. Nectopoda, con- 

 taining Carinaria ; Fam. 2. Pteropoda, containing 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 mollusca. 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. Lam. {A small nut.) Fam, Arcacea, Bl. and Lam. — 

 Descr. Equivalve, inequilateral, transverse, covered with an epider- 

 mis; hinge linear, with a series of sharp, angulated teeth, arranged 

 in a line on each side of the umbones, and a central ligamentary pit; 

 muscular impressions two, simple ; palleal impressions not sinu- 

 ated. — Obs. 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 from being confounded with any other. 

 Thirty-four figures are enumerated in the catalogue by Sowerby, 

 sen. which accompanies the Conchological Illustrations of the 

 author. The new species, to the amount of 24, have been figured 

 in parts 14 to 16, of the above mentioned work. Recent Nuculae 

 are found from the frozen to the torrid zones, and the fossil 

 species occur in nearly all the beds from the Pliocene to the 

 Carboniferous system. 



NUMMULACEA. Bl. The third family of Cellulacea, Bl. described 

 as containing shells or calcareous bodies, which are characterized as 

 discoidal, lenticular ; without the slightest traces of whorls to be 

 seen externally. The whorls are numerous, internal, and divided 

 into a great number of cells, which are separated from each 

 other by imperforate septa. This family contains the genera 



