148 The America7i Geologist. September, v».\\ 
pores as empty holes. In most places the surface of the sac 
is perfectly smooth and even, so that the location of the pores 
is not complicated or obscured by any surface ornamentation 
or irregularities. In all of these specimens the pores are well 
defined, and are located invariably on the sutures at the mid- 
dle of the sides of the hexagonal plates, and never at the 
angles. I have figured several specimens, to show the struc- 
ture in as much variety as my space would admit. Figures 
18 and 19 give anterior and posterior views of a nearly entire 
specimen, slightly enlarged, with the arms broken of¥ so as 
to expose the upper, curled portion of the ventral sac, which 
is here profusely perforated, on both sides, with large pores. 
Fig. 20 gives a posterior view of the exposed part of the same 
sac, enlarged three diameters, so that the sutures of the plates, 
and the position of the pores in relation to them, may be more 
clearly seen. Fig. 21 shows the ventral sac of another speci- 
men in a different aspect, enlarged two diameters. Fig. 22 is 
a view of another specimen, enlarged two diameters, showing 
the general form of the sac, and the pores on the posterior 
side, where some of the larger plates are stellate. In all the 
foregoing figures, except the last in part, the surface of the 
specimens is in its natural condition, smooth and devoid of 
ornament. In fig. 23, however, I show the sac of another 
specimen which I have ground ofT and polished so that all 
trace of the original surface has been removed, and the sutures 
and pores appear at a level which was perhaps half way be- 
tween the outer and inner surfaces of the plates. Here also 
the sac is seen to be thickly perforated by pores which are 
always at the middle of the sides of the plates, and never at 
the angles. 
In view of the foregoing evidence, I do not think any one 
will longer dispute the existence of pores which penetrate the 
ventral sac in certain Fistulata. In fairness, howevef, both 
to Mr. Bather and to Wachsmuth and Springer, I wish to be 
understood as limiting the statement to that contained in our 
Monograph already quoted, viz., that ''among the Poteriocrin- 
idse, in many cases, the pores pass through the test." (p. 114, 
note); and the passage in the text on the same page that in 
the Fistulata "the sac . . .is generally composed of 
longitudinal rows of hexagonal plates, which are often pitted 
