BIVALVIA. 117 



Myacites tumidus. Tab. IX, fig. 2 a, b. 



Testa subrhomboided, valvis in medio tumidd, latere antico brevi, compressiusculd ; postice 

 elongate et truncato ; margine ventrali subrecto et sinuosd; margine cardinali subrecto 

 et horizontali ; valvis lavigalis lig amentum magnum; lateribus lineis incremente confertis 

 et irregularibus. 



Shell subrhomboidal, its middle portion tumid ; anterior side short and compressed ; 

 posterior side elongated and somewhat truncated; ventral margin nearly straight and 

 somewhat sinuous; hinge margin straight and almost horizontal; the valves smooth; 

 ligament large ; series of growth numerous and irregular. 



An obtuse and very tumid surface extends obliquely from the umbones to the inferior 

 and posterior border, which renders that part of the shell more convex than is usual in this 

 genus. The anterior border slopes obliquely, but is somewhat rounded, and is moderately 

 compressed. There can scarcely be said to be a hiatus at the anterior border, and the 

 posterior border, which is somewhat truncated, has only a narrow opening. The entire 

 form is short, as much so as Arcomya brevis of Agassiz. The shortness, together with 

 the greater convexity of the middle portion of the valves, serves to distinguish it from 

 Arcomya quadrata of the same author. This species of Myacites is represented by one 

 specimen only : it has the ligament preserved, which is prominent, but not much 

 lengthened. 



Height, 16 lines; length laterally, 27 lines; greatest diameter through both the valves, 

 15 lines. 



Locality. Minchinhampton Common. 



Anatina, Lam. Ceroomya, Agassiz. 



Shell elongated; umbones mesial, small, and depressed; anterior side rounded and 

 produced ; posterior side attenuated, having a lengthened and strongly defined posterior 

 area, which has two longitudinal furrows upon its surface; no lunule. The surfaces of 

 the valves are covered with large longitudinal ridges, which are strongly marked anteriorly, 

 but are faintly traced posteriorly. There exists two depressions, more or less marked, upon 

 the side of the shell, which, originating at the umbo, diverge obliquely, and are directed 

 to the inferior border, causing that margin to undulate. These depressions, although 

 superficial, influence the direction of the longitudinal folds, make them to deviate from 

 their normal direction, and sometimes efface them altogether. The extremities of the 

 valves gape, more especially at the posterior extremity. 



M. Agassiz, judging from the contorted figure of the casts and the absence of anything 

 like a fracture, thinks that the test must have possessed considerable flexibility. 



