INIOLLUSCA 



Order II. Heteroconchice. 



Gills always lamellar, their outer surt'acc frequeiitly plaited. Hinge — 

 in rare cases (AiiotloiiUi) lost by degeneration — is heterodonl or modilied 

 from a heterodoiit condition. The mantle eilges hut rarel}' tree in their 

 whole extent; si}ihons usually present, but in some so 

 small (Integripalliata) as to cause no sinus in the pallial 

 Hue; in others (Sinupalliata) large, the pallial line hav- 

 ing a marked sinus. Anterior and posterior adductors 

 equally de\'eloped. 



Sub Order I. IXTEGRir.XLLIATA. Unioxip.t-, fresh- 

 water mussels; hundreds of species in the Alississip[n basin 

 afford material for pearl bullous. In some pearls of \alue 

 arc occasionally found. I'uio/'' AiuhIoiiIii.''' The tro]iical 

 Trid.\cx[D-K includes the largest acephalan, Tn'daaiu ,(,''',C<'-^". 

 the shell of which mav be four feel long and weigh three 

 hundred pounds. The heart shells (C.VKninw — C\irJiiim,-" 

 Si'rript's-'') and .\sr.\KTiii v, marine, and the fresh- water 

 Cycl.\diD-E (Cviliis, Pisiiliinii-'), about the size of peas, 

 belong here, as proliahly do the extinct Rinusrin.r: of the 

 cretaceous, in which the atl;ichcd right \al\e is drawn oiU 

 into a lonij; cone, the left htting like a lid over the small ca\itv. 



Sub Order II. SINUPALLIATA. The \h;Ni,Kinv with 

 swollen shells, ]'ii!iis )iicrccjiaria* quahog, I\l.\e"TKiD a:, hen 



0^7- Fio. ^;jS. 



Fig. ;,37. — .1, Sfixiciva ,ir,-lica: l\ Asl.i'i,- siilr.il.i: i' SiliiiHu rostatii ifroiu Hinnrv- 

 Goiild). 



iMo. 31S. — Ti-ri;ln »,r:',i//s, ,slii|i-wonii (after Crolilini), 'Vhv manlle opened liack 

 to the sijiiions. 1, foul; j, slu-Il; ;;, li\,T; .1, 5, viailriilc and auriilc of lu'arl; (1, 

 gonads; 7, gills; S, cloaea; c), gill ehanlln'r; 10, 11, anal and hraiuliial si|ihons; ij' 

 pallets on sijilions. 



clams, and the Ti-,ij,iNin.F. {Tell in a,''-' M'aronia*), have short sijihons. In 

 others the- siphons are so large that they cannot be entirely retrai ted within ihc 

 shell, as in the Mvm.i;, represented in all northern seas by the long ehim, M vc 

 cnnaria,'' and in the razor clams (Solknid.e; Schii, J-jisdhiltr'').' Tlir'allial 



