244 CP.ETACEOUS PELECYPODA 



lowest palaeozoic rocks, but continue sparingly up to the present time. The know- 

 ledge of the fossil forms is too imperfect to allow any certain conclusion to be drawn 

 with regard to their relation to other allied shells. Next come the A startib^ ; 

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

 period. The true astartinje appear to have been the predecessors of the Cypeinidm 

 and Venerid^^ and from the carditinm may partially have sprung up the Cardiil^^ 

 partially the Chamidje and allied forms. The Crassatellibm appear to be an 

 offshoot of the Astautibm; they first appear in middle mesozoic rocks, and seem 

 to have preceded the Mactridm, to some genera of which they exhibit, as already 

 noticed, very marked relations. The Ljjginibm and Ungulinidje are sparingly 

 represented 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 Carbubm, and partially also to the Tellinid^, have been already alluded to, 

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

 The JErycinii)^ and Galeommidje 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 Astartidm especially play a very 

 important part. 



XXIII. Family,— LUCINID^. 



The animals of the Lucinid^ are, like their shells, oval or roundish, generally 

 somewhat tumid or moderately compressed, with 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 Lucina and its allies almost obsolete, being represented by a single 

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

 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. 



