ADDENDA. 213 



The extension of the genus Trigonia to tlie Armatus-zowQ of the Lower Lias in 

 Britain could only be accepted upon the clearest evidence, which unfortunately in the 

 present instance is wanting. Entertaining doubts of the propriety of describing either of 

 the two following minute specimens as examples of Trigonia, I have omitted to figure 

 one of them, which in my opinion probably and apparently pertains to another genus of 

 Lam ellih ran cJi ia ta . 



The figure and description of T. modesta from the Yorkshire Lias as a British species 

 compel the present notice. 



The materials upon which it was endeavoured to found this species consisted of 

 two very small, incomplete, and imperfect specimens, altogether insufficient for the 

 purpose, obtained by Professor Tate in the Ammonites-armatus-zowe of the Lower Lias 

 of Warter and Robin Hood's Bays, North Yorkshire, and unfortunately no subsequent 

 discovery of the shell has taken place. I am indebted to the courtesy of the Rev. J. F. 

 Blake for the loan of these specimens. 



It appears scarcely possible to convey information by any fair delineation of the little 

 imperfect object upon our Plate ; it may represent the very young condition of one of the 

 Trigonia Costata, but even the genus is doubtful. 



This ill-defined little shell, about i^ths of an inch across the valve (the left), is in a 

 soft shaly matrix ; the test is ill preserved, and the apex has disappeared ; the area, of 

 which only the portion adjacent to the carina remains, is plain and somewhat concave ; 

 the marginal carina or posteal angle is distinct, plain, and slightly curved ; the dorsal 

 costse are small, numerous, indistinct, and nearly horizontal ; this apparently was the 

 specimen which induced Professor Tate to give the name of Trigonia modesta. The 

 small portion of the surface preserved posterior to the carinal angle is plain, a feature 

 which militates against it being one of the Trigonia Costata. 



Another speciipen, of nearly similar dimensions, and attributed to the same species, 

 appears to me to be distinct ; it is in a fragment of hard, dark -coloured, shelly limestone 

 from the same zone and locality ; the test has disappeared, but the specimen generally is 

 better preserved than the other ; it represents an ovately oblong, moderately convex 

 shell, of which the lower and posterior borders are not exposed, and the exact figure is 

 therefore doubtful ; the umbones have not much elevation, they are curved forwards, and 

 placed anterior to the middle of the valve; the anterior border is produced and rounded ; 

 the posterior border is imperfect, it has a slightly defined carinal angle, and a narrow 

 umbonal, smooth space posteal to it ; the upper portion of the valve has a numerous 

 series (about 12) of small dorsal costae, which pass forwards from the posteal divisional 

 angle and carina horizontally, and become evanescent about the middle of the valve ; the 

 an teal half of the valve is therefore without ornamentation. The an teal direction of 

 the umbones appears to be sufficient to remove it from Trigonia; it must therefore 

 remain one of the doubtful examples of the Lamellibranchiata ; possibly it may be a 

 Corbula. The posteal extremities of the few last formed costae extend nearly to 



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