78 



Trigccnta — (Bruyuiere, 1789). 



The shell is bivalve, equivalve, inequilateral, and subtrigonal ; ex- 

 teriorly it is strongly ribbed from the apex to the base (I am referring to 

 the recent species only), the furrows being well defined ; and interiorly it 

 is lined with nacre or mother-of-pearl of the most beautiful and brilliant 

 hues. But the extraordinarily intricate hinge is perhaps its most notice- 

 able feature. It is composed of two lamellif orm teeth in the right valve, 

 which are transversely grooved on both sides, and four in the left valve, 

 grooved on the one side only. These hinge teeth are constructed with 

 such delicate nicety, and they fit together with such wonderful precise- 

 ness that when the shell is closed it is difficult to open it without incur- 

 ring the danger of breaking the parts. The genus was divided by the late 

 Professor Louis Agassiz into eight sections, to the last of which — the 

 Pectinees — all the recent species belong. The first recent specimens 

 introduced to scientific notice were collected by Pe'ron during the expedi- 

 tion sent out by the First Napoleon, in the ships " Geographe" and 

 " Naturaliste," in the year 1800. Lamarck named the new and valuable 

 addition to the already magnificent collection of the Museum Rojal, 

 Paris, 1\ pectinata, but for some reason, of which I am unaware, it has 

 since received the synonymous name of T. Lamar eld from the late Dr. 

 Gray. Since then other species have been obtained, five being enumerated 

 by the Messrs. Adams in their " Genera of Recent Mollusca." One more 

 has since been added to the list by the discovery in a living state, on the 

 south-eastern coats of Australia (not the province of South Australia), of 

 T. acuticostata, originally described by Professor McCoy from fossil 

 examples. I will now proceed to a description of the several species. 



1. — Tbigonia pectinata, Lk. 



This very beautiful shell is a native of New South Wales, and is 

 most abundant in Sydney Harbour, where it is by no means difficult to 

 obtain at certain spots with the dredge. The interiors of some specimens 

 are beautifully lined with nacre of a rich silver or golden hue, while 

 others again are, if possible, still more lovely, the tint being of the richest 

 and most delicate pink. The shell receives its specific name from the 

 fact that the numerous longtitudinal ribs with which it is adorned on its 

 external surface are pectinated or combed (say like a cog-wheel). Strong 

 transverse striations traverse the intercostal spaces from the base upwards, 

 gradually becoming fainter as they reach the apex of the shell. I exhibit 

 a very large variety of this species (!) dredged at Cape St. George, a 



