222 MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 



ORDER I. TROPIOPODA LAMELLIBRANCHIATA. 



Respiratory apparatus of two unequal pairs of very 

 thin expanded branchiae, on the sides of the body, with- 

 in the mantle. 



SECTION I. MONOMYARIA. 



A single large adductor muscle, leaving a rounded impression 

 in each valve. 



FAMILY I. PECTININA. 



Animal orbicular or roundish, compressed ; with the 

 lobes of the mantle disunited beneath, or nearly in their 

 whole extent, their margin thickened, and fringed with 

 several rows of filaments ; the foot small or rudimentary, 

 somewhat dilated at the end; the adductor muscles ap- 

 proximated or united; branchiae large, united in the 

 median line, and decomposed into filaments; mouth 

 rather large, with prominent lips and four pairs of tri- 

 angular palpi. 



Shell regular, inequivalve, auriculate, internally com- 

 pact ; more or less radiatingly costate or striate, with 

 the hinge divergently plicate or toothless, and having 

 a central depression for the ligament, which is thus in- 

 ternal ; the muscular impression very large, subcentral. 



The Pectinina are all marine, free or affixed to other 

 bodies, and apparently zoophagous. Some of them 

 afford much esteemed articles of food, It is remarkable 

 that the common oyster, so abundant on many parts of 

 the British coasts, is nowhere met with on those of 

 Aberdeenshire. 



GENUS 1. PECTEN. SCALLOP. 



Animal roundish, compressed. The lobes of the 

 mantle very thin, disunited, thickened on the margins, 

 and furnished with several rows of fleshy filaments, be- 



