finest species of true LucincB in it, namely the L, tigerina, 

 L. punctata and L. iriterrupta. The circumstance that 

 has led to this arrangement has undoubtedly been the 

 proximity of the anterior lateral tooth to the hinge teeth, 

 in which character the two genera accord:* for though 

 in some Lucince scarcely any traces of lateral teeth are 

 observable, yet, whenever they do exist, the anterior 

 are placed more nearly than the posterior to the hinge 

 teeth. 



The Lucinee are equivalve shells, except one species, 

 the L. Childreni of Gray, in which the valves do not 

 appear ever to be alike, though sometimes one, some- 

 times the other is the smaller and flatter. They are 

 inequilateral; some of the species, however, the L. 

 divaricata for instance, have the appearance of being 

 equilateral. In general they are nearly orbicular and 

 lenticular, and rather depressed; there are, nevertheless, 

 some species that are very nearly globular. The teeth 

 vary much in the different species, both in number and 

 size, but not in position; in some the cardinal teeth exist 

 without the lateral, in others the lateral are more distinct 

 than the cardinal, in a few species both are strongly 

 marked, while in others there are no traces of either : 

 where they do exist, there are generally two small car- 

 dinal teeth diverging from the umbo, and of lateral teeth 

 one on each side in one valve, and two on each side in 

 the other, the anterior lateral teeth being placed near to 

 the cardinal teeth, and the posterior just behind the 

 ligament. 



Ligament external, elongated, partly concealed by 

 the inflected margin of the shell, wherefore the internal 

 tendinous portion is frequently seen lying in a deep elon- 

 gated cavity between the teeth and the hinge margin: 

 Muscular impressions two, distant from each other, the 

 anteriorf generally extended backwards and downwards 

 in the form of an elongated or ligulate band : the posterior 

 small and roundish. Muscular impression of the mantle 

 without any sinus. 



Most of the LucinsB appear to be occasionally subject 



* In the Catalogue of tbe Tanlcerville Collection we have restored these to 

 Lucina ; and we find that since the publication of that Catalogue, the same 

 observation has been made by Deshayes. 



t Called by Lamarck the posterior. 



