INTRODUCTION. 17 



Whorls of Symmetrical Univalves. 



They are disunited when they do not touch each other, as in 

 the case of Spirula (fig. 471 in the plates); but in the con- 

 trary case they are said to be contiguous. In some species of 

 Nautilus the whorls overwrap each other in such a manner 

 that the early whorls are entirely covered by the last, the 

 edges of which reach to the centre of the disk : the spire is 

 then said to be hidden ; as in the Nautilus Pompilius. In 

 Nautilus umbilicatus the spire is nearly hidden, the whorls 

 not quite covering each other ; but in the greater number of 

 the Ammonites, the largest part of the preceding whorls is 

 seen. To express the degree in which the whorls overwrap 

 each other, has caused much difficulty in concise descriptions. 

 Perhaps it would be well to apply the term spiral disc to so 

 much of the shell as is seen besides the last whorl, and to 

 describe it as large or small in diameter, compared with the 

 whole : or to say that the whorls of the spire are half, or one- 

 third, or one-fourth covered, as the case may be. 



Aperture of Symmetrical Univalves. 

 In Ammonites Blagdeni and some others the aperture is of 

 an oblong square ; it is then said to be sub-quadrated ; in Nau- 

 tilus triangularis it is angulated ; in Ammonites Greenoughi 

 it is of an interrupted oval shape, described as elliptical. In 

 the greater number of Orthocerata, it is rounded or circular. 

 The entrance of the last whorl into the aperture of some 

 rounded species of Nautilus causes it to take a semi-lunar 

 form ; if rounded at the sides it is said to be reniform or 

 kidney-shaped : if pointed at the sides it is semi-lunar ; and 

 in some species of Ammonites, it is five-sided or quinque- 

 lateral. 



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