320 Lamarck’s Genera of Shells. 
The shells of this genus are marine, and resemble the pectines 
in form, and by the internal margin being always crenate. Many 
species acquire considerable thickness by age, and experience 
such achange of form as renders it difficult to distinguish them. 
The species are subdivided into (1) Shells with distant longi- 
tudinal furrows, and frequently, in addition, delicate transverse 
or longitudinal striee—14 species; and (2) Those with promi- 
nent and radiating longitudinal ribs, with or without transverse 
strie—5 species. 
Type. Pectunculus glycimeris. (Arca glycimeris ? Linn.) 
Shell orbicular, transverse, subequilateral, longitudinally 
furrowed and striated ; when old, turgid, and very thick; ob- 
scure transverse zones. Mediterranean and Atlantic Ocean. 
In all, 19 recent species, and 9 fossil. Pl. vi. Fig. 66. 
4. Nucula*. 
Shell transverse, ovate-triangular, or oblong, equivalve, in- 
equilateral. No facet between the. beaks; line of the hinge 
broken or angular, many-toothed, interrupted in the middle by 
a pit, or spoon (cochlea) extending obliquely ; teeth numerous, 
often produced like those of the pectines. Beaks contiguous, 
curved backwards. Ligament marginal, and partly internal, 
inserted in the pit or spoon of the hinge. 
The nucule are distinguished from the pectunculi and arce, 
not only by the broken or angular line of their hinge, but also 
by the ligament, which is partly internal, and by wanting the 
facet between the beaks. They are small sea-shells, somewhat 
triangular, and more or less pearly on the inside. 
Type. Nucula rostratat. (Arca rostrata. Brug.) 
Shell transverse, oblong, rather convex, thin, transversely 
striated ; anterior side longest, attenuated, beaked. Baltic Sea. 
Six recent species, and 4 fossil. Pl. vi. Fig. 67. 
* A small nut. 
+ Beaked. Lamarck’s second species. His type is N. lanceolata. 
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