^0. 4.] BILLINGS — ON FOSSILS. 231 



which may be seen descending into it to various depths. The 

 surface layer of the bottom of the cup, extends the whole width, 

 bending- downwards a little near the margin, as in Zaphrentis, 

 and uniting with the inner wall of the cup all around. It thus 

 seems to represent one of the tabulae of a Zaphrentis. The 

 following are the principal variations observed in this part of the 

 fossil. 



1. Specimens with a perfectly smooth space in the bottom of 

 the cup ; no columella. 



2. A smooth space with a small conical tubercle near the 

 centre. 



3. Smooth with a small ridge, two lines in length and half a 

 line in heighth and width. 



4. Smooth with a compressed columella 3 lines in length, 2 

 lines in height, most elevated next to the fossette, gradually de- 

 clining in height towards the opposite side. 



5. Smooth spaces very small, columella^ a low elongated ridge, 

 with a few tubercles on its crest. 



6. Columella well developed, but with tubercles on it and 

 around it. 



7. Septa reaching the columella and more or less corrugated 

 and either with or without a columella. 



In all cases where the columella is elongated, its length extends 

 in a direction from the fossette to the opposite side. In those 

 which have the septa extending to the centre the columella is 

 often represented by a low rounded elevation. 



It is difficult, perhaps impossible, to decide whether or not 

 this group of forms, is specifically distinct fram H. excellens. 

 The greatest difference is seen in the surface characters. In 

 H. excellens the folds of growth are in general numerous and 

 angular, although some are rounded. In H. proliflca they are 

 in general few and nearly always rounded. In H. excellens I 

 have only been able to make out the septal striae distinctly in 

 one specimen. At 1 inch from the base there are 5 and at 2^ 

 inches 4 in the width of 3 lines. In II. proUjica there are 8 to 

 10 at 1 inch, and 6 to 8 at 2^ inches. 



To this may be added that H. excellens is extremely rare, 

 while H. proUfica is very abundant. 



//. proUJica is abundant in the Corniferous. I have seen only 

 one specimen from the Hamilton group. 



