MALAC0Z0A. TROPIOPODA. LAMELLIBRANCHIATA. 337 



marked with fine parallel, oblique grooves, and crenulated 

 ridges. The genital aperture is on the right side, at about 

 a-fourth of the length of the body ; and that of the alimentary 

 canal about the middle. The foot and sides are hyaline-white, 

 and semitransparent, the sides minutely dotted with opaque 

 white ; the upper surface pale greyish-brown, with a broad 

 umber-brown band on each side, the whole dotted with white, 

 most of the tubercles being tipped with a flocculent substance 

 of that colour. Along each side, in sinuses formed by the 

 margin, are six pale-grey, very soft, ramose branchke, most 

 easily separable, being merely of the consistence of mucus. 

 The whole body was enveloped in a tenacious greyish-white 

 mucus. Length two inches and four-twelfths ; length of the 

 foot an inch and eight-twelfths ; its breadth five-twelfths ; 

 height of the body anteriorly four and a-half twelfths, at the 

 middle eight-twelfths ; length of the head six-twelfths, its 

 height nine-twelfths, its breadth seven and a-half twelfths. 



The above description is taken from an individual, in per- 

 fect condition, obtained on the 30th December, 1842. It had 

 been brought up by a fishing-line from the Bay of Aberdeen. 



I have great pleasure in dedicating this remarkable animal 

 to my esteemed friend and master in science, Professor 

 Jameson. 



CLASS IV.— TROPIOPODA. 



ORDER L— TROPIOPODA LAMELLIBRANCHIATA. 



Family IV.— -Arcina. P. 207, 243. 



Genus 1. Pectunculus. Pectuncle. 



Shell suborbicular, equivalve, somewhat anisomeral ; 

 the valves convex, thick, concentrically striate, often di- 

 vergently striulate and obscurely costate, with a pilose 

 or velvety epidermis ; the umbones pointed, incurved, 

 contiguous, but ultimately separated ; the hinge margin 

 thick, broad, with an external flattened, striate space for 

 the ligament, and a curved series of alternate teeth and 

 depressions ; muscular impressions oval, distant. Pec- 

 tunculus, diminutive of Pecten. 



2f 



