GEOLOGICAL SOCIETY OF PENNSYLVANIA. 227 



scapula, which has also transverse ridges (these trans- 

 verse ridges are only perceptible when the ambulacrum 

 is removed). The extremity only of the strisB being in 

 contact with the upper part of the ridge, a vacuum must 

 remain between the strise and the margin of the scapula , 

 it is this interval which, though not round, has the ap- 

 pearance of a pore. I have endeavoured to render this 

 structure intelligible by some drawings. PI. 10, fig. 5, is 

 an ambulacrum showing its indented edge or arrow- 

 shaped striae : fig. 4, these strisB, much magnified, touch- 

 ing, with their extremities a, the ridges of the scapula 

 hhh. The interstices forming the pores are marked 

 c c c. The striae, when magnified, show a longitudinal 

 furrow in its middle, and some of them have often de- 

 pressions on their ends which indeed have more or less 

 the appearance of pores, but it is only occasioned by a 

 kind of suture which is visible in some. Fig. 6 repre- 

 sents such an ambulacrum ; we see that the striae are 

 only near the margin, and terminate at the line a a, 

 which is undoubtedly a suture ; the middle part has no 

 transverse striae, but only a longitudinal furrow. The 

 interior of this ambulacrum is longitudinally furrowed, 

 forming a number of channels which fit in similar chan- 

 nels of the interior of the body. 



It is said by Mr Say and Dr Goldfuss, that the sum- 

 mit has five rounded apertures (ovaries of Mr S., 

 or apertures through which the body takes in water, 

 of Dr G.). These openings are more complicated than 

 our authors mention. It seems tliat the specimens 

 from which they have drawn up their description were 

 not as perfect as those which I found. Each of these five 

 openings in perfect specimens is divided into two parts 

 by a septum, which is connected with the tip of the sca- 

 pula and runs thence towards the central apertu re. This 

 septum is very delicate, and has generally disappeared 

 during fossilifi cation ; but so far as I have been able to 



