742 
DR. W. B. CARPENTER AND ME. H. B. BEADY ON 
rounded ends and a much shorter conjugate axis. The two largest perfect examples 
that have come under my notice * have about the same weight (three ounces) ; and their 
dimensions represent fairly, and in by no means an extreme light, the difference in pro- 
portion alluded to. Their measurements are respectively 3^ inches by 1 inch, and 2J 
inches by 1^ inch. In other words, the proportions between the axis and the transverse 
diameter are in the long form as 65 to 20, in the short thick variety as 36 to 20. In both 
of these specimens the transverse section is circular; and others of the same form have 
proportionate dimensions ranging between the two. There are, however, some few in 
which the transverse section is not circular but lenticular (bi-convex). These are of 
smaller size, and resemble an almond in general contour (tig. 4) ; but whether the 
peculiarity is the result of compression, or is due to an inequilateral plan of growth, 
is not easily determined. I am inclined to attribute it, for reasons which will presently 
be given, to the former cause ; the more so as there is no ground for specific or 
even varietal distinction in the structure of the interior. It may be remarked in pass- 
ing, that a similar or even wider range of variation in external form exists in the iso- 
morphous genus Alveolina. In that group may be found every gradation in shape, from 
a nearly perfect sphere to a spindle with pointed ends, having a length four or five 
times as great as its thickness through the centre. Exceptional specimens of Alveolina , 
analogous to the compressed examples of Loftusia above referred to, are occasionally 
though rarely to be met with in Tertiary limestones. 
29. As might be expected from the nature of the matrix, the exact condition of the 
exterior of the test, in respect to inequalities of the surface, ornamentation, or markings 
determined by structural peculiarities, is not very readily learnt. One or two of the 
specimens which appear to have been enucleated with less disturbance of the superficial 
layers than the rest, have a series of tolerably regular furrows, nearly equidistant, tra- 
versing the shell from end to end, somewhat resembling the uncut portion of that figured 
in Plate LXXVIII., though more uniform. These depressions are not so sharply defined 
nor so deeply excavated as similar lines in the smaller Alveolince, neither have they the 
same structural significance. It seems probable that they indicate only alternating periods 
of more and less vigorous growth, or that they are dependent on external circumstances. 
Arenaceous Foraminifera generally show but little tendency to the surface-ornamenta- 
tion common in the Vitreous and Porcellaneous groups ; and there is no reason to sup- 
pose that, in the living condition, Loftusia differed materially in superficial texture from 
Trochammina. Apart from the longitudinal riblets before described, the exterior may 
have been either quite smooth or slightly granular, according to the nature of the sand 
of which it happened to be built up, and the proportion of calcareous material which 
formed the cement. 
30. Internal Structure. — Although, in general contour, Loftusia most closely resembles 
* A specimen in the Museum of the Geological Society, London, must originally Lave been somewhat larger 
than either of these ; but, as it has had sections cut from it for microscopic examination, its exact dimensions 
cannot now be obtained. 
