ABSTRACT

[1 4 ] W O R K S OF d a r w i n : v o l u m e 1 4 specimens had opercular valves, and therefore it is perhaps rash to assert quite positively that they are identical with B. tintinnabulum; but, extraordinarily variable as this latter species is, yet, after having examined so many hundreds of recent specimens from all quarters of the globe, a sort of instinctive knowledge o f general aspect is acquired, which makes me feel convinced that the fossils in question do really belong to this species. Moreover, the large shell, with its trigonal orifice passing into rhomboidal — the smooth, broad, finely porose radii, with their summits not oblique - the rather large parietal pores - and the cancellated basis, are characters which hardly concur in any other species; and those with which these fossils might be confounded, are inhabitants of distant quarters of the world. Most of the recent varieties of B . tintinnabulum, and all the fossil specimens from the Crag, can be at once discriminated from B. tulipiformis (with which, at least in the Mediterranean deposits, it is likely to be confounded) by the summits o f the radii extending from tip to tip of the adjoining compartments, and therefore not being oblique, as is always the case with the radii o f B . tulipiformis. The largest fossil specimen which I have seen is nearly two inches in basal diameter, and nearly the same in height, and therefore about two-thirds of the size of the largest living specimens.