544 H. A. NICHOLSON ON THE GASTEROPODA OF 



as these are known to me. The Gasteropoda constitute perhaps the 

 most abundant and characteristic fossils of the Guelph formation ; but 

 they are generally very badly preserved, occurring almost exclusively 

 in the form of casts or moulds, from which the actual substance of 

 the shell has usually been entirely removed. They have been de- 

 scribed by Professor Hall (Pal. N. Y. vol. ii. pp. 345-349 ; Geol. 

 Survey of "Wisconsin, 1861, pp. 34-36 ; and Twentieth Report on 

 the State Cabinet, pp. 341-347 and 364-367) and by Mr. Billings 

 (Palaeozoic Fossils, pp. 154-160 and p. 169) ; but much remains to 

 be done, owing to their poor state of preservation, before we shall be 

 fully acquainted with their characters. This is the more to be re- 

 gretted, as most of the species appear to be peculiar to the formation, 

 and have not been satisfactorily identified with similar forms in older 

 or younger deposits. 



Muechisonia Loganii, Hall. Plate XXYI. figs 3, 4. ; 



M. Loganii, Hall, Pal. N. Y. vol. ii. p. 346, pi. lxxxiii. figs. 4 «, b. 



Spire elongated, of numerous whorls (eight to twelve), apical angle 

 about 15°. Whorls strongly convex, smooth and rounded in the cast, 

 but exhibiting, in well-preserved specimens in which the shell is re- 

 tained, a distinct but low band, placed about the centre of each 

 whorl. Body- whorl not ventricose. Columella large, not marked 

 with a spiral fold. Suture deep. Surface apparently smooth and 

 without striae. 



This species is one of the commonest and most characteristic of the 

 Murchisonice of the Guelph formation ; but it is usually badly pre- 

 served, and its full characters are not yet known. In an ordinary 

 way, fragments may be recognized by the rounded, convex whorls, 

 which do not very rapidly increase in size, and are not very oblique. 

 Yiews of the interior are often obtained, and are very characteristic, 

 the columella being simply rounded and not folded as in M. bivittata, 

 whilst it is far less thick than in M. longisjoira. The body-whorl is 

 only slightly larger than the next whorl ; and I have seen no specimen 

 exhibiting the aperture. According to Hall, however, the last volu- 

 tion expands towards the aperture, and there is an expanded um- 

 bilicus. Casts of the shell are smooth; but examples in which 

 portions of the shell are preserved show that there was a flat spiral 

 band running along the centre of each whorl, the convexity of the 

 whorl being equal above and below the band. 



The size of the shell in M. Loganii seems to have been variable ; 

 but none of my specimens is quite perfect. The largest specimen 

 in my possession (fig. 3) is a fragment, broken at the top, consisting 

 of six whorls, having a length of two and a half inches, the width 

 of the body-whorl being eleven lines, and the width at the summit 

 being five lines. Other examples, however, do not appear to have 

 reached these dimensions. 



Formation and Locality. Guelph Limestones; Elora, Guelph, 

 Hespeler, Western Ontario. Also in rocks of the same age at Eacine, 

 Wisconsin, and Leclaire, Iowa (Hall). 



