INVERTEBRATES. 351 



divided longitudinally in the same direction as the basals. The inner 

 leaf is at its border as distinctly marked as the outer one ; it extends 

 not far beyond the column, but is considerably wider, and very much 

 higher than the stem joints, which are so extremely short that there 

 are eight joints to a length of half a line.' The structure is such 

 that there can be but little doubt that the inner leaf, although dif- 

 fering considerably in its dimensions from the succeeding stem- 

 joints, is actually nothing else but the proximal joint, which had 

 become enlarged and gradually anchylosed with the basals. It is 

 quite evident that the outer clover-leaf had a similar origin. At 

 the outer leaf the sides of the anchylosed joint were more extended, 

 and became almost entirely absorbed into the basal plates, leaving 

 only external marks, while at the inner ones the column structure 

 can yet be recognized, although the joint has here actually become 

 a part of the basal plates. This is the only specimen in which I 

 have observed a second series of plate-like marks, and I do not know 

 whether this instance represents an abnormal case or is owing to 

 the extreme size of the specimen; at any rate it gives an explana- 

 tion how similar marks were produced. 



In corroboration of these views I will further direct attention to 

 Pentremites abbreviatus, Hambach, a very depressed species, in 

 which the basals, even in young specimens, have an almost 

 horizontal position, and in which as might be expected, there is 

 no conical projection at the end of the basals. In this speci- 

 men, the leaf-like marks are very large and conspicuous, they 

 form a somewhat triangular, clover-leaf shaped disc, with rounded 

 extremities directed to the basal sutures. The sutures are always 

 slightly depressed, and the parts covering these depressions, 

 stand out so prominent, that it appears almost as if in this 

 case, exceptionally, the anchylosis had not been completed, and 

 in fact I found two specimens in which the upper structure appa- 

 rently had partly faUen out, leaving in its place, at the surface 

 of the basal plates, a clover-leaf-like impression. The place to 

 which the column was attached is generally well marked, and in 

 one specimen I found remains of the proximal segment which is 

 exceedingly thin and delicate, and like the anchylosed joint trian- 

 gular in form, but the points of the angles directed toward the 

 sides of the upper one. 



I think this fully sufficient to prove that the so-called supple- 

 mentary basals in Pentremites consist of a columnar joint, anchyl- 

 osed with the basals and more or less completely absorbed into the 



