230 PALEONTOLOGY OF ILLINOIS. 



must be regarded as the type of the genus, has a wide geograph- 

 ical distribution, and is common at many localities. It is known 

 from the Silurian of Russia, Gotland, Germany, New Brunswick, 

 New York, Indiana, Kentucky, western Tennessee and Minne- 

 sota. Of the specimens from American localities, those coming 

 from the western states are in most cases silicified. In a large 

 proportion of these again the skeleton has been more or less 

 completely dissolved away, leaving only the empty moulds. In 

 all of these, however, the superficial layers of spicules are pre- 

 served, but it is only in rare instances that the spicules were 

 not distorted during the changes in mineral composition which 

 nearly all fossil sponges have undergone during the process of 

 fossilization. 



In the calcareous examples of the genus, like those from the 

 Cincinnati and Lower Helderberg groups, and part of those 

 from Trenton and Niagara deposits, the minute details of struc- 

 ture are not clearly recognizable, but they serve very well in 

 giving an idea of the general plan of construction. They show, 

 for instance, that the skeleton is tubular and that the tubes, 

 which in the same species (H. intequalis) may vary in diameter 

 from 0.2 to 0.7 mm., radiate in all directions from a central 

 point. These tubes or canals enlarge very slightly toward the 

 outer surface, and new canals are continually interpolated. In 

 cross section they are obscurely hexagonal or subcircular, with 

 their walls thin and perforated by six rows of minute round 

 pores. Occasionally, also, sections of these calcareous examples 

 may afford obscure evidences of the individual spicules, but they 

 are never so conclusive as in the siliceous specimens and natural 

 moulds. 



The most perfect moulds seen by me come from the Niagara 

 beds at Spring Valley in southern Minnesota. From these I 

 have prepared a number of gutta-percha casts that for sharp- 

 ness and beauty leave little to wish for. 



Fig. 10, of the plate of cuts on page 224, represents an accu- 

 rate camera lucida drawing of a small portion of one of these 

 casts magnified 55 diameters. The specimen from which this 

 cast was prepared is fractured vertically through the center and 

 consequently exhibits the appearance of the skeleton on the 

 inner side of the broken tubes. To understand this illustration 

 it is necessary to study it in conjunction with fig. 9, which 



