622 MOLLUSCA-CLASS IL-PELECYPODA. 



modiolus, as to need no further description. The genus Mytilus is world- 

 wide in its distribution : Modiola is chiefly tropical. 



The genus Crenella is interesting on account of its hinge margin, which is 

 crenulated behind the ligament. Lithodomus, the Date- shell, so called from 

 its resemblance in external shape to a date-stone, is an interesting example 

 of adaptation to habitat. It perforates corals and the hardest rocks, forming 

 for itself a long, narrow burrow into which it exactly fits. 



The fresh-water genus Dreissensia, introduced into this country from the 

 Baltic on timber, may be placed here rather than with the fresh- water mussels, 

 despite certain of its anatomical features, till its true affinities shall have been 

 determined. 



ORDER III. PSEUDOLAMELLIBRANCHIATA. 



In this division, to which the oyster belongs, the mantle is entirely 

 open ; the foot is but little developed, and the gill filaments are reflected 

 and joined by interlamellar connections. The order is further subdivided 

 into AVICULACIA, OSTREACEA, PECTINACEA. 



Sub-order 1. Th e A VICULACE A, or swallow shells, 

 are so called from the fancied resemblance pre- 

 sented by representative forms, when the two 

 valves are spread open, to a flying bird (Fig. 9). 



They are extremely pearly shells, indeed the 



Fig. a-AvrouiAHntuNDo. P ear | ^ ter of Commerce is a conspicuous ex- 

 ample, the nacreous layer being the thickest ; 

 the middle layer of the shell is prismatic and 



consequently brittle ; the periostracum is very thin. In shape they are 

 inequivalve ; the hinge is long and straight, with few and obscure teeth, 

 or none at all, and the hinge line is usually produced on either side of the 

 umbones, forming expansions, technically known as ears. 



The ligament is external, and placed in a groove or grooves. The posterior 

 adductor is very large ; the anterior, situated close to the umbo, is small, or 

 sometimes wanting ; the pallial line is represented by a chain of small 

 isolated scars. They spin a strong byssus, which issues through a notch in 

 the right valve beneath the front ear. They dwell in temperate or tropical 

 seas, and in point of geological time are found right back to almost the earliest 

 fossiliferous rocks. The principal genera are Avicida, Meleagrina (the 

 pearl oyster), Malleus (the hammer-headed oyster), Perna, in which the 

 ligament is crumpled into numerous folds, occupying transverse grooves in the 

 thick hinge, and the common Pinna. 



Sub-order2. TheOsTREACEA,of which theoyster(0srea)is the type, present 

 some points of great interest other than gastronomic. The mantle margin is 

 double and finely fringed, forming the so-called "beard." The foot is reduced 

 to a rudiment, and there is no byssus. The anterior adductor in the adult 

 is entirely wanting ; but in the embryo, that, as in all bivalves, is the first 

 of the two adductors to be formed, the posterior developing later and per- 

 sisting, whilst the former disappears. The stage in which both are present 

 has not yet been seen in the European oyster. Traces of teeth are some- 

 times to be observed. Oysters are world- wide in their distribution, if Arctic 

 seas be omitted. 



Sub-order 3. The PECTINACEA are best known by the scallop (Pecten), of 



