378 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [DoC. 1, 



lapping each other at all, and exposing nearly the whole of the great 

 umbilicus* (fig. 2). The whorls on this, the upper, side are convex, 

 subangular a little beyond the middle, and abrupt on the margin of the 

 umbilicus ; the width of the last whorl in comparison with the pre- 

 ceding is as 7 to 4. On the lower side (fig. la) about five are visible ; 

 the inner ones are more equal in size, the second being scarcely 

 twice the width of the third. A raised sutural ridge runs round 

 the inner margin, and (unless this be due to pressure) the centre of 

 the whorl is somewhat angular also. 



The operculum (figs. 4, 5) is widely diiferent from that of M. 

 Loganif, having the nucleus produced in the most extraordinary 

 way, and spirally rolled. The appearance is that of a gasteropod 

 shell {Capulus or Pileopsis) unrolled; but by comparison with fig. 6, 

 which represents the operculum of M. Logani, the corresponding 

 arose may easily be traced. The lower edge {a) is nearly straight, to 

 correspond with the flattened base of the mouth, and the ridges 

 which correspond with the inner angle {e) and base (d) of the mouth 

 are only very much stronger than in the Canadian species. Coarse 

 rough lines of growth cover the surface, and antiquate the whole of 

 the operculum. The umbo (6) is much more produced in some 

 specimens, and incurved in a remarkable degree. All the specimens 

 have the inner process broken away. The chief figure (la) has the 

 operculum added to it in outline (1 h), to show what position it ap- 

 pears to have occupied in the full-grown shell. Fig. 3 is a restored 

 outline-view, seen edgewise. 



The resemblance of Madurea to the genus Caprotina or to Capri- 

 nella will at once be evident ; and it was to this group, the Rudista, 

 that Mr. Woodward was first inclined to refer the genus. He has, 

 however, included it in the Heteropoda as a solid form, probably 

 sedentary, and aUied to BellerojpJion. 



Ophileta compacta. pi. XIII. fig. 12. 

 (0. compacta, Salter, Decades of the Geol. SurTey of Canada, Dec. I. pl. 3.) 



As this interesting and very peculiarly-formed shell has been 

 fully described in the work above quoted, the details need not be 

 repeated here. In the Durness limestone it occurs generally of a 

 smaller size than in Canada, though we have one imperfect specimen 

 (not figured) an inch broad. The under side (a) is perfectly flat, the 

 upper (c) deeply cup-shaped, and the cast of this surface (b) is of a 

 dome-like form, with deep notches at the sutures of the whorls. 



On a careful comparison I can find no essential difference between 

 the British shell and the Canadian one, except the less rapid growth 

 of the whorls. In a specimen from Beauhamais, near Montreal, 

 1^ inch wide, the proportion of the last whorl to the preceding 

 is as 3*75 to 2. In one from Durness an inch broad, the propor- 



* Not the real umbilicus, as in other shells, but rather the sunk spire (Wood- 

 ward). The shell is not a reversed one, — the flat side is the under side. 



t ' Decades, Canadian Survey,' vol. i. pl. 1. In these figures, as also in our 

 own, the shells are represented in an inverted position — ^the characters being 

 more easily shown in that way. 



