234 BRITISH FOSSIL TRIGONI^. 



therefore sometimes not sufficiently illustrative of the fully developed growth of the 

 species.' 



The Sydney T. Jukesii is, however, nearly allied to another form which has been 

 obtained rarely in the Coralline sea at Cape York, the most northernly point of Australia, 

 separated by twenty-two degrees of latitude, and figured by Gray under the name of 

 Trigonia uniophora (' Voyage of the Ply,' 1847, Appendix, pi. 2, fig. 5). The only 

 essential difierence between the latter and the species of Sydney appears to consist in the 

 greater breadth of the posteal slope, and the greater length of the siphonal border in 

 Gray's species — features which, unaccompanied by other distinctions, can only be regarded 

 as constituting a varietal character. 



To the foregoing living Australian forms of the Pectinidce must also be added a single 

 unnamed Trigonia upon the tablets of the British Museum, remarkable for the bizarre 

 and anomalous character of the external ornaments, and especially for the characters of 

 the costse. 



With our experience of the last few years it is easy to foresee that the missing con- 

 necting links between the Mesozoic and living Trigonia may be expected to be found in 

 the Tertiary formations of i\.ustralia. 



* Specimens with individual peculiarities occur ; one of the Sydney Harbour T. Jukesii, Adams, in 

 my possession, of adult growth, has in each valve an arrest of growth near to the pedal border ; the left 

 valve has three additional narrow interstitial costse, the right valve having one such. An inordinate 

 secretion of shell substance internally causes the valves to gape at the hinge-border, exposing the hinge 

 processes with their transverse sulcations. A single, small, interstitial costa not unfrequently occurs in 

 this species. 



