4iS SCIENCE PROGRESS 



very great effort to imagine how these marginals could be 

 converted into the vertical rows of the interambulacra and 

 the pointed narrow arms, becoming curved, could have formed 

 the ambulacra." 



In the organisation of the Pelecypoda we find a feature 

 which has admitted of an almost inconceivable amount of variety, 

 both of form and arrangement, viz. the manner of the hinge- 

 attachment between the two valves of the shell. It is a matter 

 of some significance for the theory of this paper that the most 

 primitive of the Pelecypoda are those forms which exhibit the 

 Taxodont type of hinge, in which the interlocking, comb-like 

 teeth are numerous and similar in size and form. The primitive 

 nature of the Taxodont class is clearly evidenced by the fact 

 that the embryo shells of many of the higher forms {Ostreidce, 

 Pteriidce, Philobryidce, Mytilidce, etc.) pass through the Taxodont 

 stage of a more or less rectilinear or gently curved hinge-line 

 with a considerable number of teeth ; and that in still higher 

 forms {Condylocardia and Scioberetid) this Taxodont stage, 

 " present in the early embryo, is succeeded by the series of 

 folds (characteristic of the young stages of the higher Pelecy- 

 pods) that subsequently divide off into cardinal and lateral 

 teeth, thus linking the Taxodont with the Heterodont and 

 Desmodont types of hinge." ^ 



Although considerable differentiation must have already 

 occurred in pre-Cambrian times, yet the Taxodont shells (Cteno- 

 donta = Tellinomya, Glyptarca, Redonia) are relatively numerous 

 in the Cambrian period in comparison with the higher and more 

 differentiated forms, e.g. Modiolopsis, and even in the Lower 

 Ordovician a nearly similar disproportion is in evidence. It 

 is therefore apparent that Pelecypods started with a Taxodont 

 hinge ; owing to this repetition-series of similar teeth a high 

 degree of variability must have ensued, followed by a rapid 

 reduction to an optimum, i.e. to the Heterodont form of a 

 cardinal tooth and two lateral teeth. Increased specialisation 

 has led to still further reduction or even complete suppression. 

 The existing genus Nucula is one of those rare instances which 

 have retained many really archaic characteristics, not only in 

 the nature of its hinge-line, but in still possessing the primitive 

 Aspidobranch type of gills, the primitive creeping foot and the 



' Woodward, B. B , Proc. Malacol. Sac. vii. pt. 5, June 1907, p. 251. 



