100 AVICULID.E. 



SHELL tapering to a point at one end and expanding at the 

 other to an obtusely rounded or slightly truncate edge, convex, 

 with a gradual slope towards the sides, rather thin for its size, 

 more or less glossy : sculpture, 8-12 long delicate ribs on the 

 dorsal portion, radiating from the beaks, and sometimes nume- 

 rous shorter ribs on the anterior portion of the shell, extend- 

 ing to that margin ; these ribs often rise at irregular intervals 

 into vaulted prickles in consequence of an exfoliation of the 

 outer layers ; the whole surface is also covered with the mi- 

 croscopical punctures peculiar to this family : colour yellow- 

 ish-brown : margins very thin in front and on the anterior 

 side, but thickened on the dorsal side, which is straight or 

 occasionally a little incurved, and sloping gradually on the 

 other side (with the exception of a slight indentation for the 

 byssus) to a blunt angle, where it meets the front margin : 

 beaks strong and exceedingly prominent, much worn and 

 broken in adult specimens, and furnished inside with a series 

 of concentric plates, in consequence of the successive shift- 

 ings and advance of the anterior adductor muscle : Tiinge- 

 line extremely produced, occupying two-thirds of the entire 

 length : ligament horny, narrow, very strong and elastic : 

 hinge-plate folded over the ligament, so as to contain it in a 

 kind of sheath or groove, closed or firmly soldered for a short 

 distance beyond the posterior termination of the ligament: 

 inside highly nacreous and iridescent, of a darker colour on 

 the upper half than below: muscular scars very distinct. 

 L. 10-5. B. 6. 



HABITAT : Sparingly and locally distributed on all the 

 British coasts, but gregarious, from low-water mark to 

 80 fathoms, in muddy or sandy gravel. Fragments have 

 been found in the Coralline Crag. Its foreign range 

 extends south to both sides of the Mediterranean, and 

 also to the Canary Isles. It has not been noticed as 

 occurring anywhere north of Shetland. 



The " fan-mussel " is by far our largest shell, mea- 

 suring sometimes fifteen inches in length and eight in 

 breadth. It varies greatly in proportions and sculpture. 

 Specimens in some situations have a tendency to ex- 

 pand, and become distorted in consequence of their con- 

 fined position. The fry have spoon-shaped and some- 



