WOODWARD : INAUGUEAL ADDRESS. 251 



posterior portion of the test to house them, as well as the ultimate 

 abandonment of the flattened form, till finally in the deepest 

 burrowers, the Myidae and Solenidae, the closed sliell is frankly 

 abandoned, and the valves, which no longer cover the whole animal, 

 function solely as fenders against lateral pressure from the sur- 

 rounding silt. 



Facility in penetration is probably likewise the accountable cause 

 of the elongate shape of the rock-boring representatives of several 

 families of Pelecypoda. 



To the borers, rather than to the burrowers, should be referred 

 Fistulana and Brechites, with their specialized shelly tubes, which are 

 a secondary product quite distinct from the true shell. In the case of 

 the former we have had proof of its drilling powers brought before this 

 Society on more than one occasion.^ 



There are a few instances among the bivalves in which the shell 

 becomes internal (i.e. invested by the mantle) : Chlamydoconclia, Avhich 

 passes its life attached to the sheltered sides of rocks by its byssus ; 

 Ephippodonta,' which is commensal in the burrow of a species of prawn 

 {Axinus) ; Scioberetia, which is a parasite in the ambulacral zones of 

 an incubating echinoderm {2ripi/liis) ; and Entovalva, which is para- 

 sitic within Synnpta. Semper has also recorded the occurrence in 

 similar situation of another mollusc with internal shell, from the 

 Philippines,^ possibly belonging to the same genus as the last. 



No instance of a shell-less pelecypod has as yet been recorded. 



While, therefore, it is not so pronounced as in the case of the 

 Gastropoda, there is still evidence of an increasing tendencj^ in the 

 Pelecypoda towards the reduction of the shell as one proceeds from 

 the more primitive to the more specialized forms. 



One feature in connection with the bivalve shell there is, that 

 distinctly shows a tendency to simplitication, and that is the pro- 

 gressive reduction of the number of teeth in the hinge. The oldest 

 forms, such as many of the Paloeoconcha of Neumayr, the more archaic 

 living forms (NucuHdae, Arcidse, etc.), and the embryo shells of many 

 higher forms (Ostreidae, Pteriidae, Philobryidse, Mytilidae, etc.), 

 exhibit a more or less rectilinear hinge-line with numerous small teeth 

 (Taxodont). In the yet more advanced forms [Condylocardia and 

 Scioberetia) this stage, present in the early embryo, is succeeded by 

 the series of folds (characteristic of the young stages of the higher 

 Pelecypods) that subsequently divide off into cardinal and lateral teeth, 

 thus linking the Taxodont with the Heterodont and Desmodont types 

 of hinge. In these last groups the hinge-teeth progressively dwindle 

 in number, till in the most specialized Septibranchs they are wanting 

 altogether, as they are also, exceptionally, in other less advanced forms. 



Proc. Make. Soc, vol. v, pp. 25S, 345; vi, p. 185; and as an exhibit at the 



meeting in December last. 

 The anatomy of this genus formed the subject of one of the late Martin F. 



Woodward's earliest papers : Proc. Malac. Soc, vol. i, pp. 20-25. Examples 



were also exhibited at a recent meeting by Mr. Burne. 

 Reiseu im Archipel der Philippinen : Hoiothurien, p. 99. 



