MALACOZOA. 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 branchiae, 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 I. 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. 



