286 Repoet of the State Geologist. 



York city.* This specimen is adherent to the matrix along its 

 outer wall, but a portion has been broken away, and shows the 

 inclosing rock marked off by curving ridges into rhombic figures^ 

 which have the characteristic intaglio ornamentation presented in 

 the diagram, Plate YL^ figure 4. The ridges on the specimen are 

 opposite to, and continuous with, those on the matrix. It is evi- 

 dent that the outer surface must have had individual opercula or 

 a common investment whose cast is preserved on the matrix. 

 That something of the sort existed is shown by the pyritized 

 specimen also, since the radial tubes are not filled with the shaly 

 matrix (as would be the case if the ends had not been closed)^ 

 but by a white, crystalline mineral, probably celestite. 



Why the matrix appears to be continuous with the specimen 

 along the radiating ridges is a perplexing problem. Perhaps it 

 may be due to the granulo crystalline condition of the matrix. 



The specimens thus far discussed are casts and have been 

 found in the Galena limestone of Illinois and Wisconsin. 

 Another example of I^. Oweni comes from the same horizon in 

 Iowa, and illustrates a different condition of preservation. It is 

 composed of calcite and represents what has usually been called 

 the true structure of the organism, but what I consider as only 

 an infiltration product of a form like the foregoing. The matrix 

 is dolomite, softer than that from Illinois and less crystalline. 

 The curvature of the specimen affords a clue to its orientation^ 

 and the indication thus given is confirmed by the presence on 

 the concave surface of gentle elevations covered with a network 

 of intersecting channels. f Yet the pores which are said by Bil- 

 lings and HiNDE to pierce the endorhin between the radial pil- 

 lars do not exist in this specimen, but they do exist in the 

 ectorhin. 



The outer wall is marked off into rhombic plates by thin par- 

 titions of the matrix, which is continuous alike with that around 

 the fossil and that between the pillars. The points of intersection 

 of the partitions are represented by rounded pillars. A section 

 of the ectorhin showing these features is given on Plate YI, fig- 



*Ttils specimen is the one studied by Prof. Hall, and will be figured shortly in an illustrated cata- 

 logue of the Museum. 



t " The inner surface of the plate is flat ; the upper surface, or that which Is exposed in the cup or 

 disk, is oftentimes convex and deeply ridged and furrowed." (PI. XXXVII, fig. 3, c-g.) Hinde, 1884. 

 Quart. Jour. Geol. Soc, vol. XL, p. 825. 



