MALAC0Z0A. TROPIOPODA. LAMELLIBRAXCHIATA. 203 



SYXOPSIS OF THE ABERDEENSHIRE SPECIES. 



ORDER I.— TROPIOPODA LAMELLIBRAXCHIATA. 



Respiratory organs of two unequal pairs of very thin 

 expanded branchiae, on the sides of the body, within the 

 mantle. 



Lamella, a thin plate ; branchice, gills. Blainville. 



Monomtaria. A Single Adductor Muscle. 

 Family I. — Pectinina. 



Body roundish, compressed, with the lobes of the 

 mantle disunited beneath, their margin thickened and 

 fringed with filaments ; the foot very small. Shell in- 

 equivalve, auriculate, more or less radiatingly costate or 

 striate, the hinge toothless, with a central depression for 

 the ligament ; the muscular impression very large, sub- 

 central. Named from the genus Pecten. 



Gexus 1. Pectex. — Shell free, regular, inequi valve, 

 roundish, compressed, divergently costate ; umbones 

 very small, pointed ; ligament internal, trigonal. Pecten, 

 a comb. Lister. 



1. Pecten maximus. — Shell nearly orbicular, inequi- 

 valve, with sixteen convex ribs, which, with their nearly 

 equal convex interstices, are longitudinally striate ; the 

 upper valve flat. Named from being the largest known. 



2. Pecten opercularis. — Shell nearly orbicular, inequi- 

 valve, with twenty-two convex subcarinate ribs, which, 

 with their concave interstices, are longitudinally striate, 

 and transversely lamelloso-striate ; the upper "valve con- 

 vex. Opercularis, resembling a lid. 



3. Pecten vdrius. — Shell roundish, somewhat oval, 

 nearly equivalve, with about thirty-two rounded, some- 

 what compressed, irregularly echinato-laminate ribs. 

 Varius, of different colours. 



4. Pecten Isabellce. — Shell roundish-oval, nearly equi- 

 valve, little convex, with twenty-four slender, compressed, 



