37G 



ROBERT TRACY JACKSON ON THE 



line on the axis of flexion. A directly parallel case is the condition existent in bivalvu- 

 lar crustaceans. These originate by the introduction of a hinge in the median line of 

 the carapace of a normal crustacean. The ancient Ostracoda, — Leperditia, Aristozoe, 

 etc., have a straight hinge line and sub circular valves which are united on the hinge 

 line by a ligament. Thus, the resulting form of the bivalvular shell in these two entirely 

 distinct classes is strikingly similar. 



The next stage in the develojiraent of the Pelecypod shell is the loss of the early 

 straight hinge line which is superseded by a curving of the hinge line and a centraliza- 

 tion of connecting tissue and teeth of the two valves in the median line of the hinge.^ 

 The necessary form of the valves of an equilateral bivalve, fi-eely crawling on the ven- 

 tral border, is curved in section. Therefore, more oi- less arcuate iimbos would develop 

 during successive growth. This condition of growth would bring the shell to the foi'm 

 of the completed prodissoconch stage which is the i-epresentative of ancestral type rad- 

 icals in the ontogeny of the individual. A close parallel of this later stage of shell 

 growth is again to be found in bivalvular crustaceans. Some genera, as Esthei'ia, have 

 a curved hinge line, more or less ai'cuate.innbos, and bear a close resemblance to Pele- 

 cypods. In this consideration of the mechanical origin of the Pelecypod shell, especial 

 attention should be called to a highly interesting and important paper by Dr. Dall,'- in 

 which he demonsti'ates in a most striking manner the mechanical relations of the liga- 

 ment and cartilage to the hinge in Pelecyjjods, and the progressive, mechanical devel- 

 opment of systems of hinge teeth in this class. 



The form of the shell up to the close of the prodissoconch stage seems to be the nat- 

 ural outcome of the bivalvular contlition. I think the muscles which close the valves 

 may be demonstrated to be the necessary consequent of the mechanical conditions of a 

 bivalve shell. The anterior adductor muscle in Pelecypods is reduced and finally dis- 

 appears when it comes into such position that its functional activity is lost, e. g., Ostrea, 

 Pecten, Mulleria, etc. (section v). The posterior adductor also may disappear through 

 disuse. In Aspergillum, whei'c the two valves have united so as to form a simple tube,^ 

 and therefore, of course, the action of the adductor muscles is lost, the posterior adduc- 

 tor has disappeared and the anterior is reduced to a few disconnected shreds (Fischer). 

 In the phylembryo stage of Pelecypods, fig. 25, p. 300, PI. xxiri, figs. 7-11, the valves 

 are closed by a single adductor muscle, the simplest condition mechanically possible to 

 effect the desii'ed end. This adductor does not seem to be homologous Avith any muscle 

 in other classes of molluscs, and it probably developed as a necessary consequence of the 



'A straisht hinse line is cliaractcristic of many Pelecy- 

 pods and it is not easy to perceive wliat canses led to the 

 adoption of a cnrvod one in the primitive radicals of the 

 class. That it was adopted, however, is predicated by the 

 form of the completed prodissoconch as described in these 

 pages. See Plates xxiv-xxx. The prodissoconch of the 

 Unionida; has a straight hinge line, the reasons for which 

 are discnssed on p. 3G8. 



'^ On the hinge of Pelecypods and its development, with 

 an atteni])! toward a better snbdivision of the group. By 

 Wni. H Dall, Am. Journ. Sci , Vol. xxxviii. pp. Ho-WI. 

 Published since this paper was in press. 



^ The tube of this and similar genera is commonly spoken 



of as if it were something separate and distinct from the 

 shell, whereas it is a moditicd condition of the sIk'11 and 

 should be so considered. In the young of Aspei-gilUun ev- 

 ery step can be traced between the typical bivalvular stage 

 and the period wheu the two valves merge into one anoth- 

 er by concrescence and form the truly unlvalvular condi- 

 tion characteristic of the genus. This concrescence of 

 the apposed borders of the two valves in Aspergillum is 

 jiaralleU'd by the concrescenc2 of the two sides of the eni- 

 liryonic saddlc-shapcd shell of Dentalium. PI. xxiii. fig. 11, 

 which unit',' to form the tubular shell of the adult, as de- 

 scribed l)y Laeaze-Duthiers (42). 



