142 CRETACEOUS PELECYPODA 



allied genera. -The siplions are thick, of moderate length, united only at the base, 

 separated for the greater part of their length and with each of the orifices fringed ; 

 sometimes they are imited nearly vxp to the end, and in such cases the terminal 

 orifices are not fringed. 



The shells are oval or elongated, though the length very rarely exceeds double 

 the height, except in some of the fossil forms ; the structure is mostly solid and 

 calcareous, seldom thin with a somewhat nacreous like appearance inside fClementiaJ. 

 The hinge has, with slight modifications, generally two or thi-ce cardinal teeth in each 

 valve, sometimes there is one additional, and at other times one or the other of the 

 three cardinals becomes obsolete ; a lunular tooth is often present, but no other 

 very distinct lateral teeth occur ; the ligament is always external, supported by 

 strong cicatrices ; the pallial sinus usually of moderate size, occasionally only indi- 

 cated by the pallial impression being truncated posteriorly. The outer surface of 

 the shell is very variable, either smooth or striated, sometimes cancellated, but 

 rarely provided with spines. All the recent species inhabit flat shores, burrowing 

 in sand or mud, and are, according to their habitat, covered with a thinner or 

 thicker epidermis ; as a rule, they are strictly littoral inhabitants. The geographical 

 distribution is in other respects world-mde ; the eastern, especially the Indian and 

 Australian seas, are, however, the richest : some, like Cytherea, especially the Circe 

 group, Callista, Sunetta, and several others, are mostly peculiar to them ; others, 

 like Dione and partially Tivela, are prevalent in the American seas ; while only 

 few comparatively, like Bosinia, Caryatis, Gomjjliina, and others, have a more uni- 

 form distribution in the eastern and western seas. 



Dr. E. Ptomer is publishing a Monograph of the Vexerid.t: (vmder the name 

 " genus Venus"J with beautiful illustrations in the " Novit. Conchologicoe." The 

 descriptions of the species of the Cytherea group have nearly all appeared, but lists 

 of most of the other species of the family have already been printed in the " Malaco- 

 zoologische Bleetter." Dr. Pfeiffer lately also began a Monograph of the family in 

 Martini and Chemnitz' " Conchylien-kabinet ; " he has also published some 

 preliminary remarks in. Mai. Blaett. for 1868. 



In a classificatory point of view the study of recent forms has an unequalled 

 advantage over that obtained from fossil forms ; and as the above-mentioned publi- 

 cations are now in progress, I will not encroach upon the field beyond what seems 

 absolutely necessary for the correct (as far as possible) determination of the fossil 

 species, particularly those occurring in cretaceous deposits. A few indications may 

 prove useful to Dr. Eomer's and Pfeiffer's work. — I cannot omit to notice the en- 

 tirely uncustomary arrangement which Dr. Homer is introducing by his genera, 

 sub-genera, sections, and sub-sections, all of which he often designates with separate 

 " generic" names, without using them in any way in his nomenclatui-e. What use 

 then are these names? Simply to remain to be written on labels of a carefully 

 arranged cabinet ? ! If they are at all useful, they must become familiar in science, 

 and if we admit a " genus [sub-genus] species," it will probably serve all purposes 

 of science and still not lose much of its convenience. 



