386 CEETACEOUS PELEOYPODA 



XL. Family,— A VICULIJDJE. 



The mantle margins are in this family entirely open, each half consists of three 

 layers grown together ; the inner one is usually thickened towards, and fringed at, 

 the edge itself, with brown pigment cells on the inner side ; the gills are narrow, 

 the laterally reflected portions of each pair are (as in Meleagrina and many true 

 AviculceJ narrower than the median laminae, and attached to the common base by 

 strong transverse strings ; posteriorly the gills are curved or bent upwards and 

 usually quite free at the ends, neither attached to the mantle nor grown together ; 

 the palpi are of moderate size, sub-triangular, truncate below, striated on the inner 

 sides; the gills originate below them and not between them as in the ilfrjTizz)^; 

 foot small, usually somewhat attenuate at the end, below distinctly grooved, and 

 at the base provided with a strong byssal gland. The pedal muscles consist of two 

 branches, an anterior thinner pair and one posterior, much stronger, attached 

 in the convexity of the posterior adductor, which is narrow, elongated, and 

 always more or less curved. The anterior adductor is only represented by a 

 very thin muscle, placed just in front of the base of the labial palps. In 

 Avicula this muscle is generally tolerably distinct, in Melina f=^ Perna) it is 

 extremely thin, but there is sometimes (in Melina spathulata for instance) another 

 thin muscle present just behind the place of attachment of the anterior pedal 

 muscles ; this could be regarded as the anterior adductor or at least part of it, for 

 it passes from one side of the mantle direct to the other ; in Vulsella the anterior 

 adductor is barely traceable, but in Malleus it is more distinct. 



The shells of the Aviculid^ are generally inequivalve, the right valve being 

 often a little smaller or less inflated than the left one, and both are generally eared ; 

 the anterior ear of the right valve has a byssal sinus, or there is a byssal gape below 

 the beaks indicated by an insinuation of the margins of both valves ; the outer 

 layer is scaly, laminar, or partially fibrous, the inner, especially within the 

 pallial impression, nacreous; the hinge line is usually straight, the ligament 

 sub-external, placed either in one groove or in several pits ; hinge with small or 

 obsolete teeth; posterior muscular scar sub-central, elongated, usually curved, 

 anterior very slight or obsolete, at the base of the anterior wing ; pallial impression 

 entire. 



The A vicuLiD^ are mostly extinct forms ; they have been much more numerous 

 and varied in former geological periods than they are at present. They are already 

 largely represented in palaeozoic rocks. It is at present difficult to say in which 

 formation the maximum of development falls, for they are almost equally numerous 

 in each one of them, but certain genera are more or less restricted to certain 

 formations. 



The recent species of this family are inhabitants of moderate depths of the sea, 

 and chiefly of the tropical waters, where they mostly occur on coral reefs in one 

 or two fathoms of water. There are about 120 recent species on record, while the 

 number of fossil species is certainly not short of one thousand. 



