18 Prof. H. A. Nicholson on some new or 



the corresponding intervals are filled with clear calcite, crossed 

 here and there by the cut edges of the interstitial vesicles. 



In vertical sections (PL II. fig. 8) the differences between 

 L. ? Schmidtii and L. conferta are still more striking. In- 

 stead of seeing the well-marked radial pillars separated by 

 intervals filled with lenticular vesicles, as we should do in the 

 latter species, we now see a uniformly brown section, in which 

 there are no recognizable spaces filled with calcite, and no 

 vesicular tissue. All that the vertical section exhibits in L. ? 

 Schmidtii is a series of sharply undulated and exceedingly 

 thin lamellas, which appear to be in close apposition. The 

 upward bendings of these lamellse correspond with the radial 

 pillars, and the downward bendings correspond with the 

 intervals between these. Periodically a thicker and stronger 

 lamella than the rest is produced, indicating a pause in the 

 growth of the organism. The whole texture of the section is 

 also more or less obviously crystalline, though not more so 

 than one often sees in sections of Echinodermal structures. 



It need not be doubted that the peculiarities of these sec- 

 tions are in part the result of crystallization and secondary 

 change ; but I have come to the conclusion that this is not 

 sufficient to account for tlie greater portion of the remarkable 

 internal structure of this species. More particularly I have 

 come to the conclusion that no amount of crystallization could 

 account for the absence of the interstitial vesicular tissue 

 which fills in the intervals between the pillars in the normal 

 forms of Lahechia^ supposing this tissue ever to have existed 

 in the Oesel form. I have, in fact, examined thin sections of 

 specimens of L. conferta^ Lonsd,, from Dudley, in a condition 

 of intense crystallization, and I have neither observed any 

 appearances in these at all comparable with those seen in the 

 Kussian examples, nor have I ever failed to recognize in them 

 tlie radial pillars and interstitial vesicular tissue. 



If, however, the form now under consideration be really 

 destitute of interstitial vesicular tissue, and if it be really 

 composed of sharply undulated and closely approximated cal- 

 careous larainte, then it obviously can no longer find a place 

 in the genus Lahechia^ E. & H. The form which it most 

 closely resembles is one which I described from the Niagara 

 Limestone of North America under the name of Dictyostroma 

 undidatum (Pal. of Ohio, vol. ii. p. 254, pi. xxiv. fig. 6, 

 1875) . The surface, when somewhat exfoliated (Pi. II. fig. 6) , 

 has, in particular, a close resemblance to what is seen in 

 Dictyostroma undulatum. Unfortunately, the microscopic 

 structure of the Ohio species has not yet been investigated, 

 and the genus Dictyostroma cannot therefore be regarded as 



