82 CEETACEOUS PELECYPODA 



represents the calcareous tube of the FROLADACEA. 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 

 nymphae, but the cartilage is either rudimentary (Cyrtodciria) or altogether wanting ; 

 the pallial line is deeply insinuated posteriorly. 



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

 and Cyrtodaria, as recorded in H. 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 Saxicavib^ has been lately published by 

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

 cav(B, eight FanopcBce (Glycimeris), two Glycimeris, and two Cyrtodarice 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 Bocellaria or Spengleria, has been so often noticed that 

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

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

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

 BocellaricB were the history of the species unknown. 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 Saxicavib^. Thus, as regards the genus Saxicava, the place which 

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

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

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

 those of Homomya and Fholadomya, than between them and the Gastrocr^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 exception of Notomya, Anthracosia, and Qtienstedtia nearly all the 

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

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

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

 numerous than it is now. Of Faitopcea, however, there are a very large number 

 of species known from mesozoic deposits, but in external shape again they are allied 

 to numerous other genera, as Homomya, Myacites, Fleuromya, and others. Many 

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

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

 dom>ya and other Anatinidje. 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 found in the shape of casts. 



As regards species, the largest number of fossil Fanopcea probably occurs in the 

 cretaceous period, for of those from jurassics, as already stated, many belono* to the 



