82 CRETACEOUS PELECYPODA 



represents the calcareoiis tul)e of the F HO LAD ACE A. The hinge is usually 

 composed of a few teeth and pits, which occasionally become entirely obsolete in 

 full grown specimens ; there is an external ligament present supported by special 

 nymphse, but the cartilage is either rudimentary fCyrtodaria) or altogether wanting ; 

 the i^allial line is deeply insinuated posteriorly. 



The recent genera referable to this family are Saxicava, Fanopcca, Glycimeris, 

 and Cyrtodaria, as recorded in II. and A. Adams' "Genera of recent shells," with 

 the exception that the names Fanopcea and Glycimeris are here used in a reverse 

 sense. 



A list of the recent species of the Saxicavid^ has been lately pul^lished by 

 Tryon in the 4th vol. of the American Journal of Concliology ; there are nine Saxi- 

 cavcB, eight Fanopcece (Glycimeris), two Glycimeris, and two Cyi'todaricB mentioned. 



The great resemblance which exists in the form of the finger-like foot and the 

 united siphons, enveloped in a more or less coriaceous epidermis, and in the form of 

 the shell of Saxicava and Focellaria or Spengleria, has been so often noticed that 

 it is not necessary to relate it here again. Forbes and Hanley, in their history of 

 British Mollusca (i, p. 137,) state that it would often be almost impossible to dis- 

 tinguish generically between some of the varieties of Saxicava ritgosa and some 

 FocellaricB were the history of the species unknovm. The mode of life also is in 

 both families quite similar, and the only difference is the absence of a special cal- 

 careous tube in the Saxicavidje. Thus, as regards the genus Saxicava, the place which 

 is usually assigned to this family — next to the Gastrochmnib^ — cannot be better 

 justified. But when we look upon the other genera, svich as the palaeozoic Notomya, 

 the Fanopcece and others, there is a still greater relation between these genera and 

 those of Homomya and Fholadomya, than between them and the Gastroch^nidm. 

 Again, Cyrtodaria appears to indicate a natural passage to the Solenid^, and thus 

 the most natural position of the family seems to be that somewhat intermediate 

 between the last named family and the Anatinid^. 



With the excejition of Notomya, Anthracosia, and Quenstedtia nearly all the 

 fossil forms of this family appear to belong either to Saxicava or Fanopcea, and 

 the former genus is, on accoimt of its external resemblance to Trapezium, often 

 rather difiicult to be made out. It scarcely appears to have been formerly more 

 nmnerous than it is now. Of Fanopcea, however, there are a very large number 

 of species knoAvn from mesozoic deposits, but in external shajie again they are allied 

 to numerous other genera, as Homomya, Ilyacites, Fleuromya, and others. Manv 

 of the triassic species described as FanopxBa seem mostly to belong to these last 

 named genera, because they have a remarkably thin shell, such as we find in Fhola- 

 domya and other Anatinid^. Whenever we have to deal with casts, and where the 

 impressions of the hinge are not traceable, the determination must remain conjec- 

 tural. I merely need to allude to the numerous mesozoic species, which in most 

 cases are only foimd in the shape of casts. 



As regards species, the largest number of fossil Fanopcea probably occm's in the 

 cretaceous period, for of those fi'oni jm*assics, as akeady stated, many belong to the 



