116 CRETACEOUS PELECYPODA 



are usually covered, with a rather thick epidermis, and some of them live in 

 brackish water. 



7. Tellidora, Morch, 1851 (?). Shell sub-triangular, or rapidly contracted 

 posteriorly, very thin, transparent ; surface concentrically plicated ; hinge with two 

 cardinal teeth in each valve, the posterior in the right valve being usually obsolete ; 

 j)Osterior lateral teeth more strongly developed than the anterior. The type of this 

 genus is Tell, burnetii; it is a markedly different shell from other Tellince in 



. form and structure, but it passes gradually into the typical forms of that genus 

 through such species as Tell, crystallma, Chem. 



8. Tell'ina, Linne, 1758. The form of the shells varies from sub-orbicular 

 to transversally much elongated, but the posterior end is always narrower, somewhat 

 contracted or fiexured, a ridge running from the beaks to the postero-inferior end ; 

 they are either equivalve, or often the posterior end is the shorter, but sometimes 

 it is longer than the anterior ; they are always more or less laterally compressed ; 

 the cardinal teeth are generally well developed, two in each valve, and of the 

 same number are the laterals, but the latter sometimes become obsolete. It is 

 almost a general rule that the lateral teeth of the left valve are smaller than 

 those of the right. The numerous sub-genera which have been established seem 

 to me to be extremely convenient in the grouping of the many species of Tellince, 

 and. some, as, for instance, Macoma and Tellidora, are, I believe, deserving of being 

 separated into distinct genera. I may also especially mention Mcerci, Arcopagia, 

 (in parte), and Omala, which will probably have to be separated as distinct genera. 

 However, as the correct comparison of many of our fossil species still depends 

 tipon om- obtaining much better materials than those now available, it does not 

 appear desirable to introduce at present those changes indicated. 



H. and A. Adams restrict the name Tellina for a few species of the tj^e, 

 T. radiata, being transversally elongated, with sub-parallel vipper and lower edges, 

 and well developed cardinal and lateral teeth. The other sub-genera are — 



8«. Tellinella, Gray, 1852 ; the type being the well known T. virgatu, 

 posteriorly prolonged, and more or less rostrated ; the lateral teeth are stronger 

 in one valve than in the other. 



8i. Feronaoderma, Poli, 1795. Posterior side of the shell angidar, not 

 much produced; lateral teeth in one (usually the left) valve almost entirely 

 obsolete. T. i;?««ce«. Born, is the type. This sub-division is not equally charac- 

 teristic as others, and most of the species could be classed with the former. 



8e. Ifcera, H. and A. Adams, 1852. Posterior end remarkably shortened, 

 beaks directed backwards, ligament supported by small fulcra, cardinal teeth two 

 in each valve distinct, lateral smaller in the left valve, but distinctly traceable. 

 I have examined two small species from the eastern seas ; they externally very 

 much resemble a Bonux (therefore Donacilla of Gray), but the shells are thin 

 as in other Tellince. 



8d. Falceomoem, Stol., 1870. Shell elongated, hinder part shorter, the upper 

 declivity slightly convex, posterior end sub-truncate, beaks directed forwards; 



