BIVALVIA. 41 



wider and more expanded, the concentric striae are very numerous and distinct, the folds 

 of growth are few and irregular. 



Height, 7 lines ; length, 1 1 lines ; diameter, through both the valves, 4 lines. 



Localities. It occurs rarely in the shelly beds of the Great Oolite, at Minchinhampton, 

 and more frequently in the slaty or clay beds of the formation, as in the Bradford clay, 

 near Cirencester, and the Stonesfield slate of Oxfordshire. 



Mytilus imbricatus, Sow. Tab. IV, fig. 2. 



MoDioLA IMBRICATA, Sow. Min. Con., t. 212, f. 1, 3, 1818. 



Testa ovato-reniformi convexd concentrice striata, umbonibus subterminalibus arcuatis, 

 dorso antice angusto, postice planiwsculo, margine cardinali recto, parte untied lateris infe- 

 rioris brevi ventricosd. 



Shell ovately reniform, convex, concentrically striated, umbones subterminal, curved, 

 the dorsal convexity is narrow towards the anterior part, and more expanded posteriorly; 

 the hinge border is straight, its length rather exceeding one third that of the entire shell. 

 The concentric Hnes are very numerous, irregular, strongly marked and imbricated, those 

 near to the posterior side being the most conspicuous. 



Proportions of a medium sized specimen: Length, 21 lines; height, 10 lines; 

 diameter, through both the valves, 9 lines. The largest specimens acquire dimensions 

 one half greater. 



This is by very much the most abundant Mytilus of the shelly beds of the Great 

 Oolite, but the greater number of examples do not exceed 12 lines in length, and the 

 valves are constantly disunited. 



Localities. Great Oolite, at Minchinhampton ; Stonesfield slate of the same district ; 

 Bradford Clay and Forest Marble, near Cirencester; also in the upper marly beds of the 

 Great Oolite, in Northamptonshire, Bedfordshire, and Lincolnshire. 



Mytilus subl^vis, Sow. Tab. IV, fig. 19. 



Mytilus scBLyEvis, Sow. Min. Cou., t. 439, f. 3, 1823. 



— — Bronn. Letli. Geog., p. 236, t. 19, f. 14. 



_ _ Goldf. Petref. ii, p. 1/0, t. 129, f. 3. 



? Mytilus jurensis, Roemer. Oolith., p. 89, t. 4, f. 10. 

 ? — EDULis, Youny and Bird. Geol. Yorksh., t. 7, f. 10. 



Testd arcuato-cuneiformi, lavigatd, umbonibus terminalibus, acutis, dorso antice angusto 

 et convexo, postice expanse, margine superiore arcuato, inferiore concavo. 



Shell curved and cuneiform, smooth, umbones terminal, acute, the back of the shell 

 anteriorly narrow and convex, posteriorly expanded, superior margin curved, inferior 

 margin concave with steep sides; lines of growth few, distant, regular, and strongly 

 marked. 



6 



