J BIVALVES AND CUTTLES. 273 



The shell is larger and stouter than that of the edible 

 mussel, and may be distinguished from the latter by the 

 fact that the beaks are not terminal, but slightly to one 

 side. The colour is very dark blue, almost black, and the 

 shell is covered by a translucent membrane, or epidermis, 

 which in the young is prolonged into a fringe. The byssus 

 is very strong, and the animal entangles with it shells and 

 small stones, so as to form a kind of nest. It lives usually 

 in sand or mud in comparatively shallow water. 



The third mussel is a much prettier species, whose habitat 

 renders it peculiarly interesting. The edible mussel spins a 

 rope by which it fastens itself to stones or posts, the horse- 

 mussel uses its threads to weave foreign objects into a 

 protective nest, but Crenella marmorata finds shelter and 

 safety within the tests, or outer coats, of sea-squirts 

 (Ascidians). Into these it burrows deeply, so deeply that 

 its presence can only be discerned by the resistance which 

 the Ascidians offer to the touch. Ascidians of various kinds 

 are common on most shores, often growing in masses beneath 

 overhanging rocks. Frequently, also, they are torn up by 

 gales, and strewn in repulsive-looking masses along the 

 shore. If your ardour is not quenched by an unfavourable 

 exterior, and is sufficient to lead you to tear open the tough 

 cases, you are likely to find not only the curious Ascidians 

 themselves, but also one or two specimens of the pretty 

 green Crenella marmorata. It also occasionally occurs in 

 nests like those of Modiola modiolus made of shells or 

 stones. The shape is very characteristic, the shell being 

 markedly gibbous, or swollen, and rhomboidal in shape. 

 It is sculptured by fifteen to eighteen longitudinal ribs 

 anteriorly, and by twenty to twenty-five posteriorly, the 

 ribbed areas being separated by a smooth region. The 

 beaks are small, swollen, inflected, and divergent. The foot 

 is white and very long, and is used in leisurely progression, 

 as well as in secreting the byssus threads. 



In the next order, the Pseudo-lamellibranchia, are included 

 some exceedingly beautiful forms indeed, I have heard it 

 maintained that one of them, Lima Mans, is the most 

 beautiful of our common marine animals. Others of them, 

 such as certain of the scallops, have always been prized by 

 shell collectors for the bright colouring of their shells, 



