CHAPTER III. 



SPONGES, GRAPTOLITES AND 



CORALS 



FROM THE 



LOWER SILURIAN OF MINNESOTA. 



BY N. H. WINOHELL AND C. SCHUCHERT. 



Sub-kingdom PORIFERA. 



Order HEXACTINELLID^], Schmidt. 



Sub-order LYSSAKINA, Zittel. 

 Family RECEPTACULITID^l, Roemer.* 



Dr. Hinde advances the theory that the sixth or summit ray of ordinary hex- 

 actinellid sponges has in the Receptaculitidce been modified so as to form character- 

 istic head-plates. He says (loc. cit., p. 830), "In no other hexactinellid sponge, so 

 far as I am aware, are there any spicules with similarly constituted head-plates ; in 

 many, however, no sixth or summit ray is developed, but in some of the abnormal 

 spicules of the Carboniferous sponge, Hyalostelia smithii-\ Young and Young, sp.. the 

 sixth ray is in the form of a rounded knob. We have only to consider that the 

 sixth ray in the spicules of the Receptaculitidce, instead of being contracted to a knob 

 merely, as in the Carboniferous sponge, has been developed in a horizontal direction, 

 and by additions to its margins, has assumed the regular rhomboidal or hexagonal 



*The above systematic position of the Receptaculitidtc is that of Dr. George Jennings Hinde. Students desiring to learn 

 more of the detailed structure of these species and their alii nit ios to other hexactinellid sponges are referred to Dr. Hinde's 

 admirable monograph " On the Structure and Affinities of the Family of the Receptaculitida>, including t herein the Genera 

 Ischadites. Murchison (Tetragonis. Eichwald); Sphjerospongia, Pengelly ; Acanthoconla, gen. nov., and Receptaculites, 

 Defrance," Quar. .lour. Geol. Soc. London, Vol. XL, pp. 7!t5-84,S. 



In Nicholson and Lydekker's " Manual of Palaeontology ", Appendix to Vol. II, p. 1563, we learn that the'ReceptacuUtidir 

 have recently formed the subject of an important investigation by llerr Rauff (Zeitschr. d. deutschen geol. Gesellschaft. 

 bd.xli. This contribution we are unable to consult. Since, however, HerrHauff has concluded that " the ReceptacuUUda; 

 are not siliceous organisms, but that the skeleton was originally calcareous, and the siliceous examples are the result of sili- 

 fication," Dr. Nicholson is of the opinion that the family " cannot be referred to the Hexactinellid Sponges," and that " its 

 systematic position is still entirely uncertain." 



tSee Cat. Foss. Sponges, British Museum, pi. 32, fig. 1. 



[55]' 



