21.1 CRETACEOUS TELECYPODA 



lowest palfeozoic rocks, l)iit coutinuo sjiaringly up to tlie present time. The knoTr- 

 Icdgc of the fossil forms is too impcrf(!ct to allow any certain conclusion to be dra^Aii 

 with regard to their relation to otlier allied sliells. Next conic the ^.sr^Kr/^/K; 

 they are rare in palaeozoic rocks, but attain a very great development in the mesozoic 

 l)eriod. The true astartin^e appear to have been the predecessors of tlie CrpsiNiuAi 

 and F'i!ni:rid^, and from the cARDiTiN.Em.aj partially have sprung up the Cardiw^, 

 ])artially the Chamid^ and allied forms. The Crassatellid.e appear to be an 

 oll'shoot of the Astartid/e ; they first appear in middle mesozoic rocks, and seem 

 to have jireceded the Mactrid^, to some genera of which they exhibit, as already 

 nf)ticed, very marked relations. The Lucinidm and TInqulixid.e are spaiingly 

 rp[)resented in mesozoic rocks, but seem to attain an extensive development during 

 the first-half of the cainozoic or tertiary epoch, in which they perhaps had their 

 maximum, for their number appears to decrease since that time. Their relations 

 to the Cardiid.e, and partially also to the Tellinid/e, have been already alluded to, 

 but the whole of the Luclna group of shells seems to be quite distinct from these. 

 The JErycinibm and Galeommidm are the youngest of the whole order, and they 

 also possess more of the characters of the more highly organised Pelecypoda than 

 any of the families previously mentioned. They are hardly known from older 

 than cainozoic rocks, and very probably are mostly developed in the present seas. 

 More careful studies of the minute shells of the tropical regions seem to promise 

 great additions to the comparatively small number of species as yet known. 



In the list of characteristic fossils the Astartidje especially play a very 

 important part. 



XXIII. Fam ily,—L VCINIB^. 



The animals of tlie Lucinid.e are, like their shells, oval or roundish, generally 

 somewhat tumid or moderately compressed, wdth the mantle margins separated 

 in front and below, united for a short distance on the postero-ventral side, and 

 leaving a wide gape posteriorly, in which terminate the two openings representing 

 the siphons. The upper, anal or exhalant, generally is a simple oval or roundish 

 hole, the lower, branchial or inhalant, is often more or less produced into a retractile 

 siphon, sometimes provided with a fringed orifice. The edges of the mantle are 

 entire, somewhat thickened, and the leaflets more or less distinctly attached to the 

 inner surface of the valves, which on this account attain a rough appearance. The 

 gills are very large, occupying the greater part of the sides, thick, according to 

 Deshayes, sometimes composed of two to five layers of blood-vessels, but all united 

 to a single fleshy lobe. The mouth is, according to the same author, very small, 

 and the palpi in Ztcclna and its allies almost obsolete, being represented by a single 

 labial tubercle. The same observation has been made on the animal of Corbis. 

 The foot is cylindrical or vermiform, very elongated, and without a byssus, hollow 

 internally, more or less easily retractile into the shell. 



The shells all agree in their rounded or oval shape, solid structure, internally 

 rugose surface of the valves, large oval muscular scars and entire pallial sinus. 



