20 Proceedings of the Royal Society of Victoria. 



are about twelve in number. The longer persistence of this 

 character in the fossil is of interest, but my series is too small to 

 say whether we have here a constant diflerence from T. lamarckii, 

 the species to which it is said to be most closely allied. 



Though I have a fairly large number of specimens of T. 

 howitti all have abraded umbos, but a specimen in the cabinet 

 of Rev. A. W. Cresswell shows that, like its congeners it also 

 had discrepant ornament in its youth. 



I was anxious to find out if the remaining recent species 

 exhibited the same chai^acters, and, as our Melbourne Museum 

 did not contain the material, I wrote to Mr. C Hedley, of 

 Sydney, asking him to supply my deficiencies. It appeared that 

 recently Mr. Hedley's attention had been turned to the same 

 point, but he informed me that, as far as he had been able to 

 find out, the facts had not been recorded, and he generously 

 urged me to publish my results. 



The drawing is from a young specimen of T. margaritacea 

 given me by Mr. J. H. Gatliff, and to his kindness I also owe au 

 example of T. lamarckii, and the opportunity of examining a 

 series in his cabinet. 



If we assume, as seems probable, that concentric ornament is 

 the more archaic form, and due to the accentuation of the 

 incremental lines, and that radial sculpture originated as a 

 breaking up of these lines, then we find, in the Pectinatae, the 

 archaic sculpture persisting longer in the anterior region of the 

 shell, and the more modern first appearing towards the posterior. 

 In the older members of the group, as typified by T. subundulata, 

 this archaic ornamentation persists throughout life towards the 

 anterior end, while in the more modern members it disapj^ears at 

 the close of the brephic stage, being entirely replaced by the more 

 modern form of sculpture. Similarly, in the posterior region the 

 signs of old age first make their appearance, and from thence 

 gradually pass towards the front. 



In the older members of the Pectinatae some interesting 

 variations occur. Thus, in T. subundulosa some individuals 

 even from the same stratum, show that the radial lines have 

 transgressed beyond the marginal carina and appear as grooves 

 crossing the concentric costae, thus breaking up the costae into 

 oblong, flattened nodules. The extent to which this occurs is 



