98 MOLLUSCA FROM THE GREAT OOLITE. 



An acute angle passes from the umbo obliquely backwards, separating a narrow area 

 from the remainder of the surface ; the concentric striae are continued upon the flattened 

 posterior area. This little shell is one of the most abundant in the formation ; its con- 

 centric stria? are very frequently not preserved, and the valves are never found in opposition. 

 The test is thick, and the characters of the hinge strongly marked. 



Height, 3 lines ; lateral diameter, 4 lines. 



Localities. Minchinhampton Common, and Eyeford, Gloucestershire. 



Xk.era Ibbetsoni. Tab. XII, fi°r. 9. 



Xe.era Ibbetsoni, Morris. Geol. Son. Journ., 1853, p. 341, pi. 14, fig. C. 



Testa subghbosd, pyriformi, subaquivalvi, striata; umbonibus magnis submedianis; 

 latere antico rotundo ; postico producto, bicarinato, mbrostrato; basi curvato ,• lateribus 

 plici-s regularibtts inconspicuis .• micleo lavi. 



Shell subglobose, pyriform, subequivalve, striated ; umbones large, rounded, mesial ; 

 anterior side rounded ; posterior side produced, attenuated, and bicarinated, the anterior 

 carina acute ; lower margin curved ; the sides with regular, slightly marked plications ; 

 nucleus smooth. 



A very convex and nearly equivalve shell, with an acutely marked angle upon the 

 posterior attenuated slope ; anterior side rounded. The nucleus has the posterior extremity 

 compressed, short, and truncated. It ranks as one of the most rare productions of the 

 Lincolnshire beds. 



Height, 9 lines; length, 11 lines; diameter through both the valves, 8 lines. 



Localities. Danes Hill ; Essendine, and Ketton quarries. Dedicated to Capt. L. B. 

 Ibbetson, F.R.S., in whose company it was first noticed, much compressed in the clays 

 above the Ketton Oolite. 



Family — Myad.e. 



iously to stating our views upon this extensive family, we desire to record our 

 obligations to Agassiz, for his important work, 'Etudes Critiques,' which exhibits a large 

 amount of patient research, of critical sagacity, and original views. The author has, 

 however, candidly admitted that his work is imperfect in certain of the details — that facts 

 are sometimes wanting or insufficiently known, and consequently that the genera proposed 

 by him are probably not all of equal value. The subject, indeed, is connected with 

 difficulties of more than one kind, and of such a nature, that subsequent observers might 

 be expected to differ in their estimates of the value of the several generic distinctions pro- 

 posed by M. Agassiz, and might even determine to discard some of them altogether. The 

 length of time which has elapsed since the publication of the ' Etudes Critiques' has been 

 sufficient for the accumulation of many additional facts tending to render our theoretical 



