1^6 CRETACEOUS PELECYPODA 



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

 as their hinges have been examined) can be sub-divided into five well characterized 

 genera and several sub-genera. The most ancient forms are those of Baroda and 

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

 may belong to them. In the cretaceous beds the Baroda forms are most numerous, 

 and a third type apparently identical with that classed under Amygdala, and 

 characterized by radiating striae, comes .next to Icanotia ; true Tapes also occur 

 here. Amygdala and most of the other recent types of tapijsinjs are met with in 

 the tertiaries.* 



About 85 recent species, including two 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 VeneridjE, (see further on). 



b. 8uh-family— VENJERINjE, 

 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 in their entire 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 with epidermis ; the lio'ament 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, all 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 Venus, Mercenaria, &c., have been restricted and in those which have 

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

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

 the right one, the superior and inferior margins of w^hich are sometimes (though 

 not often very distinctly) elevated, thus adding two teeth to the one strictlv 

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

 each valve three distinct cardinal teeth proper; one posterior elongated, directed 

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

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

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

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

 the analogy between the two seems to me to have been slightly misunderstood. 



^ I can only say tliat if Tapes (Amygdala) decussata from the Paris basin is really identical with the recent 

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

 recent forms does not, however, support this view. 



