Anomia.] PELECYPODA. 811 



Fam. ANOMIIDJE, Gray. 



Animal with a small foot with a terminal infundibular cavity ; 

 muscles of the byssus leaving one or more impressions on the left 

 valve. 



Shell variable, irregular and inequival've when sessile, byssiferous 

 when young ; in most genera the byssus becomes modified to a calci- 

 fied or horny plug passing through a foramen in the attached valve, 

 and fastened to other objects, a condition which may be permanent 

 or transient ; area small, amphidetic ; ligament amphidetic, more 

 or less internal, supplemented by an internal resilium, for which the 

 crura serve as chondrophores, ali- or multi-vincular ; hinge usually 

 edentulous, rarely rugose, with amorphous interlocking rugosities ; 

 posterior adductor small, subcentral, in the sessile forms reinforced 

 by the pedobyssal muscles, which are modified for service as adductors. 



Fossil. ? Devonian ; Jura to Recent. 



KEY TO GENERA. 



A. Processes of right valve not united ; left (convex) valve with 



3 central muscle-scars . . . . . . . . ANOMIA. 



B. Processes of right valve united ; left (convex) valve with 2 



central muscle-scars, the upper one radially striate . . PLACUNANOMIA. 



Genus i. ANOMIA (Linne), Miiller, 1770. 



Anomia, Linne, Syst. Nat., ed. 10, 1758, 700 (in part) ; 0. F. Miiller, Prodr. 

 Zool. Dan., 1776, xxx, 248. Type : .4. craniolaris, L. Echion + Echio- 

 noderma, Poli, 1791. Cepa (Hwass), Humphrey, 1797. Fenestella. 

 Bolten, 1798. Anomya, Agassiz, 1839. Diploschiza, Conrad, 186') ; not 

 Anomia, Da Costa, 1776, nor of Bolten, 1798. 



Animal having the mantle completely open, the margins with a 

 short double fringe ; labial palps striated on both sides ; 2 pairs of 

 branchia? on each side, without appendages, of the same size, and 

 united posteriorly ; the outer branchiae incomplete and free ; foot 

 small, cylindrical, terminated by a cavity ; near the foot a lamellate 

 surface is secreting a calcified plug which is nothing else but a calcified 

 byssus, attached to the left valve by 2 or 3 muscles ; there is one 

 central small adductor muscle of the valves, consisting of two ele- 

 ments ; ovarium extending into the inferior substance of the mantle ; 

 sexes distinct ; rectum not passing through the heart- 

 Shell irregular, subcircular or oblong, often imitating the object 

 to which it is attached ; thin, subtransparent. the interior somewhat 

 nacreous, inequivalve ; left valve entire, convex, having a ligamental 

 fossette below the summit, and bearing 4 muscular impressions a 

 small one for a muscle of the byssus near the fossette, 3 others at the 

 centre of the valve in a space (disc) circumscribed by a line ; the 

 2 upper impressions are those of the muscles of the byssus ; the infero- 

 po?terior impression is that of the adductor muscle of the valves ; 

 right valve flat, operculiform, having a large sinus on its upper border 

 through which the plug-like byssus passes, and whose extremities are 

 not soldered together ; one of them, posterior to the sinus, is re- 



