1865.] BILLINGS — SILURIAN AND DEVONIAN FOSSILS. 195 



Receptaculites, only that it is hexagonal. The plates of all the 

 spicula enlarge until all come into contact, and thus an outer 

 tesselated integument corresponding to the ectorhin is formed. In 

 this stage a section through the seed-like body shews an inner 

 integument (or endorhin), and an outer plated integument (or 

 ectorhin), the two being separated and at the same time connected 

 by the pillar-like cylindrical shafts of the spicula representing the 

 tubes of Receptaculites. The space between the tubes is, according 

 to some authors, filled with a gelatinous silicious matter ; but 

 Bowerbank says he did not detect this substance in the specimens 

 examined by him. This little sac or cell is a Receptaculites in 

 miniature, and it is also one of the embryonic stages of a sponge. 

 When we consider that the full grown and adult individuals of 

 many of the long extinct tribes of animals never attained in their 

 structure a more advanced organization than that exhibited by the 

 embryos of orders living at the present day, it does not seem sur- 

 prising that we should find in the palaeozoic rocks a sponge which 

 although often of large size, never became more highly developed, 

 than is the recent genus Spovgilla, when it has only advanced to 

 the sac-like stage above described. It is not intended to assert 

 here positively that Receptaculites is a sponge, or to determine the 

 question of its zoological rank one way or the other, but only to 

 direct attention to such peculiarities in its structure as appear to 

 me worthy of being taken into account in the investigation. 



Genus Pasceolus, Billings. 



13. 14. 



13. P. Halli. From the Middle Silurian, Anticosti. 14. P. globosus. 

 Trenton limestone, Ottawa. 



The fossils of this genus are of an ovate or globular form cov- 

 ered with an integument of small polygonal plates (?) and with one 



