8 CANADIAN FOSSILS. 



It is clearly distinct from, though closely allied to, the great 

 Maclurea magna of the Chazy limestone, which may be seen so often 

 in the paving-slabs and door-ways of New York, and which has been 

 well figured by James Hall. 



The genus is by no means rare in the old deposits of the Silurian 

 seas, and apparently there were several very distinct species, some 

 of which are yet unpublished, and one, M. Peachii (Salter), which 

 has the operculum extravagantly elongated and curved, occurs in 

 plenty in a rock very like the Canadian limestone, and full of similar 

 types, in the north-west angle of Scotland. It has been figured, 

 and its locality described, by Sir R. I. Murchison, in the Quarterly 

 Journal Geological Society for 1S58. Another is plentiful in the 

 Llandeilo limestones of the south of Scotland. 



Description. — M. Logani, when perfect, is fully three and a half 

 inches wide, and is conspicuous for the great flatness of its lower or 

 whorled side, and the fewness of its whorls, for, if we except one or 

 two minute inner ones, there are but two or three distinct whorls, 

 which diminish so rapidly in breadth that the outer is at least thrice 

 the width of the preceding ones in succession, and greater than that 

 of all the inner whorls taken collectively : in M. magna it is greatly 

 less than these. The whorls are very gently convex between the 

 sutures, which are sharply marked though not deep, and are closely 

 striated by regular sharp-arched lines of growth. The sides of the 

 whorl are steep, pyramidal, the depth exceeding the width of the 

 whorl, and are furrowed by a number of deep grooves, sometimes 

 16 or 17, a few of which are interlined with smaller ones. Occa- 

 sionally seven or eight only are present, or a deep one (figure 3) 

 occurs at a short interval ; but this may be the result of injury. 



The base itself is smooth, or with faint concentric strias only, and 

 the umbilicus* rather abrupt and very narrow, not above one third 

 the width of the whorl, and with a rounded edge. The shell is solid, 

 nearly a line thick. 



The most singular part of the shell is its operculum, sometimes 

 fixed, as in figure 3, in its normal position, at other times drawn 

 within the shell. It is exceedingly solid, the successive layers are 

 subspirally arranged, tiling over one another, and are antiquated in 

 growth. The nucleus is near the inner and lower angle of the mouth ; 

 in old shells it is pushed further out, and becomes the apex of a very 

 solid short cone, one face of which lies close upon the inner flat 



* Used in a false sense, since it is the perforate spire. 



