146 CRETACEOUS PELECYPODA 



Thus tlie known recent and fossil species of tapesin^, (the latter only so far 

 as their hinges have heen examined) can he suh-dividcd into five well characterized 

 <j-enera and several suh-genera. The most ancient forms arc those of Buroda and 

 Icanotia, and there are even some Jurassic species known as Psammohlcc which 

 may helong to them. In the cretaceous heds the Baroda forms are most numerous, 

 and a third type apparently identical wdth that classed imder Ami/tjdala, and 

 characterized hy radiating striae, comes next to Icanotia; true Taj)es also occur 

 here. Amyodala and most of the other recent types of tapesin^ are met witli in 



the tertiarics.* 



Ahout 85 recent species, including tAvo or three not yet described, are known, 

 and by far the greatest number of them are from the eastern seas. Because the 

 determination of the fossil forms is as yet in many respects doubtful, I shall enu- 

 merate the cretaceous species referable to the sub-family subsequently in conjunction 

 with other VisNERiDyE, (see further on) . 



b. 8uh-family—VENERINjE. 

 The animals have a strong, prolonged, laterally slightly compressed foot, 

 pointed at the end, thick at the base, but without a byssus, sometimes with a small 

 or nearly obsolete byssal groove ; the mantle has occasionally slightly crenulated 

 margins, the outer edges being separated m their entu-e length ; the siphons are 

 moderately produced, united at the base, separated towards the ends, the lower or 

 branchial siphon is usually somewhat longer and thinner than the upper, which is 

 thicker ; both have more or less fringed orifices. The shells are oval, oblong or 

 sub-trigonal, mostly stout and robust, externally smooth or variously ornamented 

 with concentric and radiating ribs, often covered wdth epidermis ; the ligament is 

 external, the muscular impressions strong, but the pallial sinus generally faint, with 

 a small or obsolete posterior sinus. The hinge is composed of a series of strong 

 teeth, aU of which may be considered cardinal; for there seems to be a perfect 

 parallelism between the distribution of the hinge-teeth in those forms for which 

 the names Venm, Mercenaria, &c., have been restricted and in those Avhich have 

 been called Cytherea, f=31eretrixj, Circe, &c. In both there is generally a strong 

 lunular tooth developed in the left valve fitting into a corresponding cavity of 

 the rio-ht one, the superior and inferior margins of Avhich are sometimes (though 

 not often very distinctly) elevated, thus adding tw'o teeth to the one strictly 

 cardinal of the right valve. In Cytherea, and also in the Circe group, Ave have in 

 each valve three distinct cardinal teeth proper; one posterior elongated, du-ected 

 backwards, and parallel to the ligamental ridge ; tAvo are anterior, resembling one 

 bifid one, placed just below the beak, or very nearly so ; to this comes the luiuilar 

 tooth and a pit in the other valve. When we compare with this the hinge of Venus 

 and its allies, it looks at the first sight very different, but apparently only because 

 the analo"-y betAVcen the two seems to me to have been shghtly misimderstood. 



* I can only say that if Tapes (AmygdalaJ decussa/a from the Paris basin is really identical with the recent 

 species, it would show a remarkable consistency in the character uf tapesi.\^ during various times. The study of the 

 recent forms does not, however, support this view. 



