134 CEETACEOUS PELECYPODA 



Again, some of the fossil species in form resembling Donax have been called by 

 palaeontologists Donacites, but the name cannot with our present knowledge of the 

 respective shells be allowed to have any signification in conchological literature. 



4a. Serrula, Chemnitz, ? Shell elongated, sub-equilateral, anterior side 

 inflated, somewhat flexuous, rounded, posterior equally or slightly less produced, 

 sub-compressed, with a rounded plicature, angularly rounded at the end ; fulcra 

 not very prominent ; surface concentrically and partially radiately finely striated. 

 The well known European D, trunculus, Linne, may be considered as the type 

 of this sub-genus. Some of the species like J), navicitlus or gracilis appear to 

 be more brackish forms and in form and dentition allied to IpMgenia. The 

 lateral teeth are small in the right valve and obsolete in left ; there is besides 

 a small anterior cardinal in the right valve, not usually noticed in other forms 

 of typical marine Serrulce. 



4ib. Heterodonax, Morch, 185? Shell broadly triangular, somewhat rounded, 

 sub-compressed, smooth, covered with epidermis; hinge-teeth distinct in both 

 valves. Deshayes' _D. ovalimts is the most characteristic form of this sub-genus. 



iiC. Capsella, Gray, 1854 (?) Shell very much elongated, posterior side 

 much shorter, oblique, with a rather sharp plicature, sub-angular at the end; 

 surface covered with a thin epidermis. D, acutangulus, Desh., is about the most 

 typical form of this group. 



4r/. Hecuba, Schuhm., 1817. Shell sub-triangular, sub-equivalve, anterior 

 end narrowly rounded, posterior more or less produced, concave at the upper 

 marginal slope, supplied with a very sharp plicature, angular at the end ; lateral 

 teeth strong in both valves, and besides that the right valve has near the anterior 

 end of the slope a marginal groove, in which fits the corresponding edge of the 

 left valve. The best known species of the present type is D, scortum, Linn. ; 

 it represents a better marked sub-division than any of the other sub-genera. 



5. IpJiigenia, Schuhm., 1817. Shell elongated, of tellinoid form, but 

 rather inflated ; hinge with two cardinals in each valve, the anterior in the right 

 the smallest, lateral teeth obsolete or nearly so ; sometimes indicated in the 

 left valve; pallial sinus deep. The species of IpUgenia are estuary shells, 

 covered with a greenish epidermis ; they are at present only known from America 

 and the Senegal, but one species, and this a very large one, certainly occurs in 

 the Ganges delta. They form a natural passage from most of the marine 

 Donaces to the more brackish or fresh water types which follow next. Chenu 

 accepts for these shells Lamarck's name Capsa, quoting at the same time 

 after d'Orbigny some fossil forms which have nothing at all to do with this genus, 

 Deshayes as well as d'Orbigny having taken the name Capsa in a totally 

 diflferent sense from Chenu. 



6. Flscheria, Bernardi, 1860, (Monog. des genres Galatea et Fischeria, 

 Paris). Shell broadly triangular, rather high, covered with epidermis; hinge 

 with two cardinals in each valve, the posterior one in the right valve small, 

 marginal ; two small laminar, lateral teeth in the right valve ; pallial sinus deep^ 



