18 Proceediuys of the Royal Society of Victoria. 



T. acuticostata. M'Ooy; T. howitti, McCoy, and T. murravica, 

 Tate, the latter being very closely allied to T. howitti.^ 



The recent species are all characterised by the possession of 

 radial ornament alone, concentric ridging being absent, and this 

 feature also occurs in the more recent of the fossil forms, namely 

 T. acuticostata, T. howitti and T. murravica. The remainder, 

 the older members of the group, have a discrepant ornament, 

 radial ridging appearing on the posterior third of the shell and 

 concentric on the anterior part. It is this discrepant ornament 

 which seems to ally them with the more ancient fossil forms 

 more particularly with Jurassic ones. A close alliance, however, 

 as we have seen, is denied by Lycett, who gi'ouped one of the 

 most typical of those with discrepant ornament, namely, T. 

 subundulata, with the modern radially ribbed Pectinidae. Since 

 Lycett wrote, our other two older tertiary species have been 

 added to the list and belong to the same group. 



An examination of the young shells of several of the radially 

 ribbed species shows the justness of Lycett's grouping, for, in all 

 that I have been able to examine, discrepant ornament occurs. 



In T. margaritacea the prodissoconch measures 02 mm. in 

 breadth, and is smooth. 



Trigunia margaritacea, umbonal portion of right valve 

 of a young specimen. 



This stage is marked off from the succeeding one, the brephic, 

 by a margining ridge. From this till the shell is about 1 mm. in 

 breadth it has discrepant ornament. On the anterior half of the 

 shell a series of concentric ridges is developed, which are sharp, 

 almost lamellar, marked off by broad, shallow, concave grooves, 

 and separated from one another by some four or five times the 

 width of the ridges. Of these ridges some eight or nine occur. 



1 Trans. Roy. Soc. South Australia, vol. xix., 1S95, p. 202. 



