LAMELLIBRANCHIATA. 265 



GOSSELETTIA, Barrois. 



GOSSELETTIA TRIQTJETRA. 



PLATE XXXI, FIGS. 9-17; AND PL \TE LXXXVII, FIG. 1-2. 



Pterlnea trlqueter, Coxrad. Geolog. Surv. N. Y. : Ann. Rept., p. 116. 1S3S. 

 Mytilarca ti'iquder (Conrad), .Hall. Prelim. Notice Lam. Shells, etc., p. 22. ISfiO. 



Gfo.f.^elettia triqiicti-a ((.'oxrad). Hall. Pal. N. Y., vol. v. i)t. 1. Plates ami Explanatiims : PL 31, 



Hfi's. 1)-17. .Jan., 1SS3. 



Shell of medium to largL' size ; both' triangular, oblique ; in young shells the 

 height and length are nearly equal, in older shells becoming more elongate ; 

 anterior margin truncate, flattened on the ventral side; posterior margin 

 broadly rounded. 



Valves equal, very convex, sub-angularly gibbous along the ventral side. 

 The convexity or breadth of the closed valves is about equal to the height 

 of the shell. Ventral side vertical. Dorsal side sloping very abruptly in 

 young individuals, and less abruptly in older shells. 



Hinge-line straight, nearly or quite equalling the height of the shell. 

 Beaks anterior, strong, elevated and incurved. 



Test thick, marked by fine concentric lines of growth which are crowded 

 and much elevated on the anterior side. In older specimens the stritB 

 become irregula-r and lauiellose toward the basal margin. 



Ligamental area comparatively wide and very finely striated longitudinally. 

 Cardinal teeth three, strong and somewhat diverging. Lateral teeth elongate 

 and extending nearly to the posterior extremity of the hinge-line. The 

 anterior muscular scar is sub-circular, deep, situated near the inner border of 

 the cardinal plate and truncating the anterior cardinal tooth. 



A medium-sized specimen has a length of 58 mm., height 4o mm., breadth 

 41 mm. A larger individual has a length of 80 mm. and height 55 mm. 



The older shells present some similarity to M. {Plethomytilus) oviformis, but the 

 ventral angular slope is characteristic. In young specimens the triangular and 

 extremely gibbous form distinguish it from all other species here described. 

 34 



