﻿CONCLUDING SYNOPTICAL OBSERVATIONS. 233 



which are distributed in the shallow Coralline seas girding the eastern coasts of Australia, 

 throughout more than thirty degrees of south latitude, from Cape York to Tasmania. 

 Known, in one instance only, as a species of the sea, in other instances as denizens of 

 land-locked waters, or of brackish waters in tidal rivers, these TrigonicB do not appear to 

 form varieties at any one locality. The differences between these species or varieties, 

 chiefly founded upon their exterior forms and ornaments, will have to be con- 

 sidered and determined in connection with their aspects over the entire marine area 

 occupied by each form. Eventually it may in this manner be possible to ground our 

 knowledge of this section upon the living, in comparison with the Australian Tertiary, 

 forms of the genus, and thus to legislate, with greater authority, upon the questions of 

 species and varieties of the Peciinida. 



In making comparisons of the species or varieties of the living Peciinidce, we may 

 select two Australian forms nearly allied, which have, of late years, become well known 

 from their abundance ; one of these is the Trigonia of that land-locked, fine expanse of 

 water constituting Sydney Harbour, and the mud of its tributary, the Paramatta river, 

 it is the T. Lamarckii of Jenkins and the T. Jukesii of Adams. It has been regarded 

 by some naturalists as a variety only of a larger and nearly allied Tasmaman form, to 

 which Lamarck's name, Pectinata, is now exclusively applied. The latter is abundant, 

 buried in the black mud of the Launceston river, or in the tidal portion of it, the brackish 

 water of which extends up the course o*f the channel for many miles. Separated from 

 the habitat of the other shell by eight degrees of latitude, the difference of form, although 

 only inconsiderable, is very persistent at each locality, and is instantly detected in the 

 adult stage of growth. T. pectinata has the lesser convexity; the umbones are smaller 

 and more oblique ; the length of the siphonal border is greater ; the costai over the 

 valve' generally are less numerous ; they are closely arranged anteally, but become 

 widely separated over the middle of the valve, the spaces between them increasing 

 towards the posteal slope ; the costellse upon the slope are small and inconspicuous, the 

 costae near to the border and throughout the circumference of the valve degenerate in 

 their crenulations into closely placed imbricated lamelkne of growth, which are obscured by 

 the greater development of the epidermal tegument ; a similar feature is seen in various 

 Mesozoic species, and notably in the Neocomian Trigonia nodosa. 



The Sydney species has a similar kind of ornamentation ; but, having larger and less 

 obhque umbones, it is more globose ; the hinge-area, corresponding vidth the escutcheon 

 in the Mesozoic forms, is larger, and slightly excavated, and its ornamentation is more 

 prominent than in T. •pectinata ; the costse over the shell generally have smaller inter- 

 stitial spaces, their crenulations become more square or flat-topped over the middle of 

 the valves and near to the lower border. The siphonal border is always shorter and 

 more perpendicidar, forming a more considerable angle with the hinge-border ; this 

 feature from its prominence would alone be sufficient to separate the two forms. The 

 young shell is usually more orbicular, having the siphonal portion less developed, it is 



