a3 Transactions of the [Monts Bone a tera 
the microscopist. Now if I am right in my observation, this classi- 
fication of the markings of P. angulatum as oblique—that is 
wholly oblique—is erroneous, as I find the markings, strangely 
enough, to be of a mixed nature, and it is to this peculiarity I wish 
to call attention. I have made out most satisfactorily, that in the 
said P. angulatum and P. quadratum the ends of the valve, for 
about one-sixteenth of its length, have not any markings but such 
as are distinctly rectangular, markings of the character in fact of 
those seen in the second division, as P. hippocampus, P. Balticwm, 
&ec., the striz being, as in these last, farther apart transversely than 
they are in the direction of the axis of the shell. It must not be 
assumed that these markings are either accidental or few in number, 
for not only have I found them to be always present, but in what 
perhaps I may be permitted to call a normal or symmetrical valve, 
I have counted 13 rows of dots from the mid-rib towards each 
edge, while I made out the number as 32 to 50 dots arranged in 
these lines between the obliquely-marked portions of the valve and 
its end. In the sketches which accompany this paper, I have drawn 
with the camera lucida an entire Lorica, and indicated exactly the 
bounds respectively of the oblique and rectangular arrangements. 
The larger drawing (Fig. 1) represents the appearance of the end of 
one of these chosen Pleurosigmata under the most favourable con- 
ditions of light and definition, and when, as is by no means always 
the case, the valve is symmetrical. This symmetry is so little con- 
stant, that I should venture to say, that the silicious envelope seems 
subject to what gardeners usually call “sports,” that is, that in all 
the ends having the rectangular markings I have attempted to de- 
scribe, scarce any two of the specimens would be exactly alike. For 
instance, the portion of valve on one side of the mid-rib would be 
rectangularly marked to double the extent of the other, in which 
case I generally found that the other end of the shell repeated the 
peculiarity. On a few occasions, I have found the curious “ sport” 
(Fig. 2) which I have indicated in one of the sketches, where the 
longitudinal close series of dots forming striz, bend at right angles 
without a break, and so become transverse; the next series follow 
the example, till the whole set has somewhat the appearance of a 
set of carpenter’s squares. I think I am not deceived in my obser- 
vation, that these rectangularly arranged “dots” are really larger 
in size, or aS we are accustomed to call it “ coarser,” than the usual 
and well-known “dots” which are arranged obliquely, and they are 
not quite so easy to resolve, and to deprive of spectra or shadows. 
Now I do not want anybody to take this peculiarity of structure 
in our old friend P. angulatum for granted; I trust that each 
microscopist will examine for himself, it being only fair to say, how- 
ever, that to make out this structure well, will require the very best 
appliances, and no small amount of care in the use of them. I feel 
