78 
Dr. Gregory, on the Fost'tertiary 
and not divided by a raphe. But it varies in breadth, even 
in the Surirella, and is probably only narrower there because 
the form is oval : and as to the form, the Surirella occurs 
frequently of so broad an oval as to be all but orbicular. 
Then, as we have seen, the Campylodiscus is most frequently 
orbicular, and not saddle-shaped. Indeed, as the Surirella 
varies much in form, being often panduriform, I think it 
quite possible that it may also occur orbicular, or as nearly so 
as the Campylodiscus I have figured. 
The question here arises — Is it possible that these two 
forms should belong to the same species ? And I am not pre- 
pared to answer this question in the negative. It is true that 
at present they are in dififerent genera ; but I would remind 
the student of these forms, that not long ago, nay, perhaps 
even now, Campylodiscus spiralis, as we call it, figures in 
Continental works as Surirella flexuosa. There is, therefore, 
a natural affinity between the genera, if really dififerent ; and 
in that case, C. spiralis and the form here described may be 
forms exhibiting the transition from one genus to another ; it 
seems, however, more probable, that there may be no essential 
distinction between these genera. I have omitted to state 
that both forms, the Surirella and Campylodiscus y are equally 
frequent in the Glenshira Sand. (293.) 
42. Campylodiscus bicruciatus, n. sp. This beautiful form 
is by no means so frequent in the deposit as the preceding. 
Fig. 42 represents it. 
Form nearly orbicular. Median space square, marked with 
two sets of coarse lines, at right angles to each other, giving 
to this part a fenestrate appearance.^ From each side of this 
square proceed two triangular or conical prongs, or prolonga- 
tions to the inner margin, which their points touch. These 
prongs are strongly striated, each pair transversely to its 
course. From the corners of the square proceed, from each 
corner four lines to the inner margin, which, as in the pre- 
ceding species, is formed of small semicircles, convex outward, 
and joining the points of the sixteen lines and those of the 
* I have some reason, from recent observations, to suspect that the 
fenestrate aspect may depend on the fact, that the two valves are so placed, 
that the lines in one are at right angles to those in the other. If so, this 
is probably the normal position, for I have not seen any example of the valves 
lying obliquely. Of course, in the supposed case, the single valves will 
exhibit only four prongs ; and this I think I have seen. But, if this be the 
case, it is not easy to see how, if tlie cross bars are seen through the upper 
valve, the open spaces should come out as clear as if the two valves lay 
with the bars coincident, Mr. West informs me that he has seen this form 
in an exotic marine gathering. 
