Till; ST KM AND CALYX. .'A 



though tin- curvature of these sutures before they reach the lower edges of the 

 radiala is not showu. 



We are inclined to think thai the alteration in the course of the interbasal sutures 

 and the apparent presence of the so-called supplemental basals, are alike due to a 

 secondary limestone deposit upon the upper part of the stem, which spreads over the 

 lower portion of the cup, and is segmented in a slightly different manner from the 

 parts which it encloses (Fl. V. fig. 25). One or two instances of secondary deposits 

 of this nature have already been noticed among the Neocrinoids. Thus in a stem of 

 the recent Pentacrinus Naresicmus 1 several of the lower joints are covered by a 

 sheath of limestone which almost obliterates the cirrus-sockets at the nodal joints; 

 while the suture-lines between its component parts do not quite coincide with those 

 between the stem-joints which it encloses. Then, again, on one side of a specimen of 

 the fossil Millericrmus Pratti 2 two large stem-joints are visible just below the basals ; 

 but they are broken away on the other side, and it then becomes manifest that they 

 enclose a central core of much thinner and narrower joints, which seem to extend 

 upwards to the basals, and to have been continuous with those lower down the stem. 

 There appears therefore to have been a secondary deposit of limestone outside the 

 upper stem-joints, which divided up into segments not corresponding with those 

 enclosed by it. In this case the interbasal sutures are not continued downwards on 

 to the apparently large stem-joint beneath them. But in other respects the resem- 

 blance between this structure and the supposed supplemental basals of the Blastoids 

 is a very striking one, and we are inclined to think that the presence of the latter is 

 due to some secondary deposit of this kind. Another reason for thinking so is, that 

 they are not unfrequently marked by more or less distinct circular lines which might 

 be mistaken for the interarticular sutures of the stem (PL V. fig. 25). 



It is, we think, this structure which is described by "Wachsmuth 3 in the following 

 passage. " In one of my specimens, which I take to be an extremely large specimen 

 of Pentremitcs Godoni, I find within the clover-leaf another leaf-like structure, but 

 of less width, and beneath it eight joints of the column, 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 differing 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 anchy- 



1 Zool. Chall. Exp., Part xxxii. 1884, pp. 328, 329, pi. xxx. a, figs. 4, 5. 



2 Quart. Journ. Geol. Soc. 1882, vol. xxxviii. p. 33, pi. i. fig. 21. 



3 Proc. Davenport Acad. Nat. Sci. 1885, vol. iv. p. 81. 



