1865.] BILLINGS — SILURIAN AND DEVONIAN FOSSILS. 191 



therefore, that Eichwald's genus Tetragonis was founded on a 

 species of Receptaculites, with the ectorhin removed. The genus 

 Ischaditcs also exhibits very similar markings, as may be seen by 

 comparing the figures of/. Kcenigii (Murch.), on pi. 12, ' Siluria,' 

 and the following of R. Canadensis. 



The specimen represented by fig. 10 has been figured by me in 

 the Geology of Canada, p. 304, under the name of Ischadites 

 Canadensis. It is the cast of the inner surface of the ectorhin, 

 and differs remarkably from the similar specimen of R. Oweni 

 (ante, fig. 7). It is deeply pitted all over with sub-quadrangular 

 or rhomboidal depressions, the form of each cavity being such as 

 would be made by the impression of a small four-sided pyramid. 

 In the bottom of each is a small rounded orifice, from which 

 radiate three grooves to three of the angles of the square. These, 



11. 



10. 



10. R. Canadensis (Billings). 11. R. Iowensis (Owen). 12. R. Jonesii 

 (Billings). 



I have not the least doubt, are the grooves of three of the stolons. 

 The absence of the fourth stolon may be accounted for in this 

 way. Among the detached silicified tubes of R. occidentalis 

 which are found in the sediment left at the bottom of the vessel, 

 after dissolving specimens of limestone holding these fossils in 

 acid, numerous specimens have been collected with only three 

 stolons in contact with the plate or at the end of the tube, but 

 with the fourth a small distance from the end. It is evident that 

 in casts of the inner surface of the ectorhin of specimens with all 

 the tubes thus constructed, there would be only the three grooves 

 of the terminal stolons visible on the surface, the fourth being 



