CANADIAN FOSSILS. 9 



surface of the whorl. Two curved furrows radiating from the nucleus 

 divide the surface into three areas, less distinct in the mature shell. 

 Inside, a thick compressed process (a) takes its rise beneath the 

 nucleus, retaining its place near the inner angle of the mouth, even 

 in the adult shell, when the nucleus itself has removed further out 

 (figure 6). The process is as broad as long, and on its oblique free 

 margin it is roughened and grooved for the attachment of muscles. 

 Another attachment, similar but much less prominent, exists at the 

 inner and upper angle (b), and a line of minute prominences partially 

 connects the two. 



These processes have their analogues in the internal ridges of the 

 operculum in Nerita and some other genera, and in that of the small 

 Pteropod shell, McGillivrayia (Forbes), but no recent shells are 

 known in which they are equally produced. 



The view above expressed of the shell being a reversed one in 

 which the spire is deeply sunk, in the so-called umbilicus, and the 

 latter expanded and flattened so as to appear like the upper side in 

 ordinary shells, will not appear unlikely, if comparison be made with 

 Ophileta (plate 3), in which there is a similar condition of the spire. 



Moreover, this view has the advantage of rendering it unnecessary 

 to suppose an operculum arranged on a different plan to that of 

 ordinary univalves, since no case is known in which the nucleus is 

 placed at the upper angle of the mouth, though some few have it 

 external. The genus Atalanta however, says M. Woodward, has the 

 spire and the operculum both sinistral ; hence it is possible that 

 Maclurea might be an opposite case, in which it is dextral in both. 



The alliance of this heavy shell with the lighter and fragile 

 Nucleobranchs would seem much more unlikely, had we not such 

 solid forms as Bellerophon to compare it with. But there are no 

 other genera with which it can be better associated, since Euompha- 

 lus has quite a different operculum (multispiral), and no opercula are 

 known in Ophileta or RapMsloma, depressed forms resembling Maclur< a, 

 but which are probably, with Murchisonia, members of the families 

 Ianthinidas or Trochidce (see plates 2, 3). Prof. Forbes believed there 

 was some affinity between Maclurea and McGillivrayia, above men- 

 tioned, a minute spiral Pteropod with the nucleus of the operculum 

 external, and with a process beneath it. But this affinity is not so close 

 as that suggested by Belleroplion with the Nucleobranchs, and we must 

 therefore leave it for the present where the judgment of Mr. Woodward 

 has placed it, and regard Maclurea as a Heteropod with a heavy shell, 

 and probably stationary or nearly so on the bottom, seeing that its 



