122 MYTILIDiE. 



and those on each side of it in the same family are 

 never capable of being precisely defined, it is obviously 

 undesirable to encumber our system of classification 

 with more divisions and names than are necessary to 

 separate and recognize the species contained in such 

 group. 



1. Modiolaria marmora'ta*, Forbes. 



Mytilus marmoratus, Forb. Mai. Mon. p. 44. Crenella mctrmorata, F. & H. 

 ii. p. 198, pi. xlv. f. 4. 



Body thick, pale-yellow: incurrent tube large and bag- 

 shaped, formed of two pendulous puckered flaps of the mantle, 

 mottled with purplish- or reddish-brown and white flakes ; 

 margin plain : ecccurrent (or anal) tube small and conical, of 

 the same colour as the larger tube, farrowed at the base, and 

 having the mouth or opening fringed witb four or five minute 

 dark cirri : foot white, very long and almost cylindrical, with 

 a deep byssal groove : byssus semitransparent but strong. 



Shell oval, very gibbous and obliquely angular, rather 

 thin, glossy, and somewhat iridescent : sculpture, 15-18 lon- 

 gitudinal ribs on the anterior side and 20-25 on the other 

 side ; these ribs or striae occupy the two sides only ; the inter- 

 mediate space is smooth, with the exception of very fine and 

 crowded transverse lines, which traverse the whole surface 

 and give the ribs an appearance of being minutely punctured : 

 colour yellowish, irregularly mottled with purplish- or red- 

 dish-brown spots or blotches, sometimes forming zigzag 

 streaks, occasionally of a uniform orange hue : epidermis light 

 green : margins rounded and obliquely truncate at the ante- 

 rior end, nearly straight and slightly gaping on the ventral 

 side, wedge-shaped or bluntly pointed at the posterior end, 

 whence they slope backwards with a gentle curve to the dorsal 

 angle or hinge-line : byssal sinus long and narrow : beaks 

 small, swollen, inflected (as in the genus Verticorclia), and 

 divergent, placed near the anterior margin: ligament thick 

 and strong, yellowish-brown, nearly concealed in the embrace 

 of the hinge-plate : hinge-line slightly curved, occupying the 

 whole of the dorsal margin : hinge-plate strengthened by an 

 internal rib and receiving the ligament in a shallow groove ; it 



* Marbled. 



