n6 PALEONTOLOGY 



Allied to them is Pyrula (Cret -Rec.), another of the 

 forms, now exclusively tropical, found in the British 

 Eocene : a pear-shaped form with very short, obtuse 

 spire, and very large aperture with thin outer lip. 



Another group of siphonostomes is represented by the 

 whelks, Buccinum and Nassa (Tertiary and Recent), the 

 former of boreal and the latter of general distribution. 

 These are elongate-oval imperforate shells with spires of 

 moderate length, wide apertures and short anterior canal, 

 ornamented by coarser vertical and finer spiral lines. 

 In Buccinum the canal is in line with the long axis of the 

 aperture ; in Nassa (Fig. 34, A) it makes a considerable 

 angle with it. The latter genus is also distinguished by 

 crenulations on the interior of the outer lip. Another 

 allied genus is Neptitnea [Chrysodomus], a boreal form, 

 nearly smooth, notable for its left-handed species which 

 abound in the Red Crag (Fig. 35, a) 



Murex (Cret.-Rec.) differs from the whelks, firstly in 

 having a very long and narrow canal, the edges of which 

 bend over until they all but meet (split-tube structure^, 

 and a very rounded aperture; secondly, in the thicken- 

 ing of the mouth-border, from which and along the 

 anterior canal there often arise long spines which repeat 

 the split-tube structure. These mouth-borders persist as 

 varices, often of great regularity in arrangement, there 

 being exactly three to a whorl, so that those of successive 

 whorls continue one another in a vertical (or very slightly 

 oblique) line up the spire ; it has been stated that in 

 these forms one turn of the spire represents a year's 

 growth. In other cases the varices are more numerous. 

 In Typhis (Eoc.-Rec.) the anterior canal and spines 

 become perfectly tubular. Murex is now widely dis- 

 tributed except in the cold seas, Typhis is more closely 

 confined to warm waters, but, like so many others, is 

 found in the British Eocene. 



