32 CUETACEOITS PELECYPODA 



II. Order. MYACEA. 



The only characters, common to the largest number of Pelecypoda referable to 

 this order, are a short lingui- or digiti-form foot and elongated, partially, or entirely 

 united, siphons, into which, however, the branchia are but slightly, or not at all, 

 prolonged. 



The valves are generally well developed, covering the whole animal ; they are 

 more or less solid, gaping posteriorly, attached to each other by an internal carti- 

 lage, and joined by a hinge with few, or sometimes obsolete, teeth ; not unusually 

 there is also an external ligament present, but it rarely excludes the presence of the 

 cartilage. Most of the species burrow in sand and some also excavate solid objects ; 

 the entire shell is often covered with an epidermis, which usually extends over the 

 united portion of the siphons, and in this respect seems to replace the calcareous 

 tube of the PHOLADA CBA, which is especially apparent in those cases in which 

 it becomes, (as in the typical genera My a and ^/^a^Ma),— almost perfectly coriace- 

 ous, and non-retractile within the valves. 



The families which may be considered as constituting the order, are the Myibm, 

 (conBULiN^ and myinm), Mactridm, (LxiTRAniiNM and mactrinjeJ, Anatinidje 

 (fandoeinje, tsraciinje, and anatininjbJ, Saxicavid^, Glauconomyid^., and Solen^ 

 iDJB. The various genera of these families are somewhat differently allied to the 

 previous and the next order : for instance, Sphenia, of the Myib^, is in an equal 

 degree related to the Gastroch^nid^, as is Saxicava, (belonging to the Saxicavidm), 

 The LUTRARimm, of the Mactribm, are undoubtedly most closely allied to the Mym 

 and some species of the ANATimBM, while the mactrin^, as restricted, in external 

 aspect, much resemble some VJEJNBEAOUA. Again, among the Anatinidm the 

 PholadomycB recall several forms of Psolabin^ ; some shells of the thraciinj^ n.VQ 

 externally almost undistinguishable from others belonging to the Tullinidm, while 

 genera, like Novaculina (of the Sojlunib^) are truly intermediate between Tellina 

 and Solen, as regards shells as well as animals, for they have long divided siphons, 

 like Tellina, but a short club-shaped foot, like Solen. 



These various relations make it really almost impossible to bring the fami- 

 lies into an arrangement, by which a gradual passage from one to the other 

 would become clearly exhibited, but the arrangement here adopted may for the 

 present, probably, best answer our as yet imperfect knowledge of the order, espe- 

 cially when we pay due attention to the very large number of fossil forms. Eor 

 this deficiency is largely due to the latter, the greater number of which we only 

 know from their external shape ; rarely is there an opportunity offered to examine 

 the internal characters of the fossil shells. The order includes, however, some of the 

 oldest known fossil forms. In the palaeozoic times the species were not very 

 numerous, but they increase considerably in the lower mesozoic epoch and attain 

 their maximum of development in the middle and upper mesozoic times. The 

 tertiary forms are less numerous in species, though more varied as regards generic 

 types ; many of the older forms have then almost entirely disappeared, and thus 



