A KKVtsloN iw niK At STKAl.lAN ll.'InAiNA — IIF.M.KY. 165 



A itidii'j^ (li'ed^iims li'oiii I he (iicitt IJiiriier Ivcfl, iilMnit t lii' lal it ikIp nl 

 Cainis. I was mi lorliiiiaU! as to Hud ttirue iiiiiintc valvt-s ol a 'I'lidnrmi tuo 

 yiiuiiif to name s|itH'iHeally. Tlie sinallest is 1'5 in height, and 1.1 mm. in 

 length; llie next - liiir'' a"d X'^b mm. lon^-- ( I'l. xxxiv., lig. l!^) ; tlie 

 iHiiresl 'l-l liigli, -"l- long and 0() mm, dfcp (IM. xxxiv., lig. 1(>) ; tim 

 height and lengtli being measured at liglil angk's to each other. in the 

 Vouiigest stage, the dorsal, anterioi' and [)us(erior-ventraI sides are nearly 

 equal, t'orming ati equilateral triangle. As the shell grows it becomes 

 more wedge-shaped, tlie height rapiilly increases and the posterior slightly 

 outgiows the anterior end. There is jio gape, but the anterior margin is 

 beset with denticules. The anterior side is flattened and sculptured by 

 Hner riblets. The back of the valve carries tive broad cresceutic ribs 

 furnished with scales, their interstices have Hner riblets. 'JMiei'e is a small 

 dome-shaped piodissoconch. The hinge line is straight. The shell is too 

 transparent to show the muscle scars. In the left valve are two extended 

 posterior teeth, in the right one and in each valve a single minute 

 cai'dinal. 



The orientation of this irregular shell has been u matter of such 

 difticulty that the valve which one author, Vaillant, terms the left, is by 

 another Avriter, Lacaze-Duthieis,'^ called the right. 1 would proceed by 

 assuming that the hinge is, as usual, dorsal, and that the beaks are directed, 

 as Usual, anteriorly. Then it will follow that the pedal gape is, as Lacaze- 

 Uuthiers argued, on the anteritir siile and that the richly coloured mantle 

 exposed by the animal living in the burrow, is [)OSterior. 



Systematists, following Lamarck, have arranged Tr'nhnnni next to 

 Canlinm. It is now contended that a more natural allotment Avould 

 associate TriJucna with the mytiloid CanUtido'. Both are rock dwellers 

 and one species of ('m-dihi was even named triihinivide>> because of its 

 resemblance to the big clams. Such classification better explains the 

 direction of evolution followed by 'irldncun. Beginning with a normal 

 Vfiiericardin, the path of distortion leads flrst to an elongate form, such as 

 r. hinjidii, liamarck (I'urditu iucrnssato, Sowerby) in Avliich the anterior 

 side has become shorter and the anterior adductor muscle has ascended. 

 Another step is represented by such forms as Beym'im '<e»ii(>rhicidut<i, liinnc 

 or ('iirdlfK ma^.'ficiista, Lamarck. Here the anterior extremity has pushed 

 out into a lobe, followed by an insinuation at the byssal slit, the anterior 

 adductor muscle has made a further ascent towards the umbo. The furrow 

 of the lunule, almost swallowed beneath tlie over rolling umbo, is hardly 

 visible except as a notch in the hinge line. The final stage in elongation, 

 compression and twisting is re[)resented in Tridiirim. At last, the anterior 

 lobe of the mytiloid Curditidic has been squeezed out of existence, the 

 byssal gape has been enlarged to form the pedal orifice of Trldnciin, the 

 anterior adductor muscle has moved on, under and past the umbo while 



* LacaZf-DuthitTo— Arcliiv. /j.jnl, Exper. (3), x., 1902,]). 2Vb. 



