On Species ofHippothoa and Alecto. 123 



XV. — Descriptions of Species o/'IIippothoa and Alecto from 

 the Lower Silurian Rocks of Ohio, tcith a Description of 

 Aulopora arachnoidea, Hall. By II. Alleyne Nicholson, 

 M.D., D.Sc, F.R.S.E., Professor of Biology in the College 

 of Physical Science, Newcastle-on-Tyne *. 



[Plate XI.] 



The fossils upon which the following communication is founded 

 were in the first place kindly submitted to me for examination 

 and description by Mr. U. P. James, of Cincinnati, an ac- 

 complished and experienced observer, and a studious worker 

 in the richly fossiliferous Silurian strata of the State of Ohio. 

 Subsequently I had the opportunity of visiting Ohio personally, 

 and I obtained a large additional series of these forms at Cin- 

 cinnati and at Waynesville. They constitute a small group 

 of organisms which may be advantageously considered to- 

 gether, though differing considerably in their nature. The 

 first of them is the Alecto inflata of Hall, which is an un- 

 doubted Polyzoon, though certainly referable to another genus. 

 I have examined veiy carefully a number of beautifully pre- 

 served specimens, and am able to give a more complete descrip- 

 tion of its characters than has yet been published. Three 

 species (viz. A. frondosa, A. auloporoides, and A. confusa) 

 appear to me to be undoubted examples of Alecto, and they 

 all would seem to be new. Lastly, I have appended a descrip- 

 tion oi Aulopora arachnoidea, Hall, because this form, whilst 

 seeming to be a genuine Aidopora, presents certain striking 

 points of resemblance to Alecto auloporoides, with which it 

 might readily be confounded. 



1. Hippothoainjlata, Hall. PI. XI. figs. 1, 1 a. 



Alecto iiiflata, Hall, Pal. X. Y. vol. i. p. 77, pi. xxvi. figs. 7 a, 7 6. 



Polyzoary creeping, adnate, branched, and forming a close 

 but irregular network. Branches linear ; cells uniserial, pyri- 

 form, each springing by a contracted base directly from the 

 cell below ; about four cells in the space of one line. Cell- 

 mouths smaller in diameter than the expanded end of the cell, 

 subterminal, and placed more or less distinctly on the front 

 face of the cell. 



Though in some respects resembling some of the species of 

 Alecto, I think there can be no hesitation in referring this 

 beautiful species to the genus Ilippothoa, with which it agrees 



* Coniraunicated by the Author, having been read at the meeting of 

 the British Association at Belfast, before Section C. 



