BIVALVE MOLLUSCS 



337 



order as the Marine Mussels, and agree with them in the character 

 of the gills. The shell is somewhat rectangular in form, with a 

 long hinge-line possessing many small similar teeth. The foot 

 has a flattened under-surface, an exception to the general rule 

 among bivalves. The group is represented by species in all parts 



of the world, and some of 



1 It ifv JT^to the individual species have 



a very wide distribution, as 

 in the case of the British 

 species Area lactea. 



In the Scallop Family 

 we have represented a third 

 order of bivalves which pos- 

 sess gills more complicated 

 than those of Ark- Shells and 

 Edible Mussels, but less so 

 than in the Fresh -water 

 Mussel and associated 

 families. There are several 



Fig. 197. Edible Mussel (Mytiltts edulis) 



Fig. 198. Pilgrim Scallop (Pecten Jacobeeus) 



species of British Scallops belonging to the genus Pecten, in which 

 the shells are fan-shaped. The Pilgrim Scallop (Pecten Jacob&us) 

 (fig. 198) is interesting as the source of the " cockle-shell " which 

 the mediaeval pilgrim to the Holy Land wore in his hat. Only 

 one adductor muscle is present, the equivalent of the posterior 

 one in the families so far mentioned. In some of the species the 

 valves of the shell are equal according to the general rule, and 

 in these the animal is able to swim by flapping them. In other 

 cases, of which the edible Scallop commonly seen in fishmongers' 

 shops furnishes an example, the animal is sedentary, and during 

 life rests with its right valve below, this being well curved, while 

 the upper or left valve forms a flat lid. A byssus is present, 



VOL. I. 



22 



