MARINE MOLLUSCS 223 



may be distinguished from the left by its greater convexity, and 

 by the presence of a notch under the anterior ear. The hinge is 

 straight, with a very narrow ligament, and the internal cartilage 

 is situated in a central pit. 



The mantle of the animal is free, with double margins, the 

 inner of which forms a finely fringed curtain all round, and on this 

 curtain are a number of black eyes surrounded by very fine 

 tentacles. The gills are in the form of very thin crescents, and the 

 foot is shaped like a finger. 



Although the majority of scallops are inhabitants of tropical 

 seas, several species are to be found off our coasts, where they 

 range from depths of about four to forty fathoms, and the empty 

 shells, often in the most perfect condition, are frequently found on 

 the beach. 



The Common Scallop (P. maximus) is largely used as food, and 

 is therefore a common object in the fishmonger's shop. Its colour 

 is very variable, and the shell has equal ears and about twenty 

 radiating ribs. The Quin (P. opercularis) is also an important 

 article of diet in some parts. 



Perhaps the prettiest of the British species is the Variable 

 Scallop (P. varius), so called on account of the very variable colour 

 of the shell, the ground tint of which may be almost anything 

 between a very pale yellow and a dark reddish brown, and this 

 is irregularly patched with some lighter colour. The chief dis- 

 tinguishing features of the species are the spiny projections of the 

 numerous ribs, most prominent near the margin of the valves, and 

 the presence of a permanent byssus, which, in other species, occurs 

 only in the young. Three of the species named above are shown 

 on Plate V. 



We may also mention the Tiger Scallop (P. iigrinus), the 

 radiating ribs of which are sometimes slightly formed, and which 

 has only one ear in each valve ; and P. pusio, in which the adult 

 shell is often greatly altered in form. 



It may be noted, in conclusion, that all the species of this genus 

 have the power of swimming rapidly by flapping their valves a 

 mode of locomotion very common among the bivalves especially 

 during an early stage of their existence. 



Before passing on to the univalve molluscs, we must refer 

 briefly to a group of animals that are enclosed in bivalve shells, 

 and which were once included with the Mollusca, but are now 

 made to form quite a distinct group by themselves. We refer to 



