SUB-GENUS MONOTRYPA. 183 



surface of attachment. The species seems to be constantly 

 parasitic upon the tapering ends of minute species of Orthocera- 

 tites, and the coralhim is constantly cylindrical in form, some- 

 times becoming slightly fusiform or clavate. In general the 

 Orthoceras has disappeared, and the space which it originally 

 occupied is filled up with calcite or with the matrix, or may 

 be simply hollow. 



Corallites thin - walled, polygonal in shape, and generally 

 nearly equal in point of size, their long diameter being, as a 

 rule, between i-iooth inch on the one hand, and i-8oth inch 

 on the other hand. Surface either smooth, or sometimes 

 showing groups of corallites of rather larger size than the 

 average, these groups being elevated to form low and ill- 

 defined tubercles. Tabulae absent throughout the greater part 

 of the tubes, but apparently not wholly wanting in any given 

 section, an occasional complete partition being developed here 

 and there in the deeper parts of the corallum, or, still more 

 frequently, close to the mouths of the tubes. 



Obs. — The external form and superficial characters of ]\L 

 clavacoidea have been accurately described by Mr U. P. James 

 (Cat. Lower Sil. Foss., 1875), to whose kindness I am indebted 

 for specimens of this, as of many other types. I am not aware, 

 however, that its microscopic structure has hitherto been noted. 

 In its mode of growth and its resulting form, M. clavacoidea 

 is quite peculiar. The colony is never large, its length vary- 

 ino" from half an inch to at least two inches, and its width from 

 three or four to eight lines ; while its form is constantly cyl- 

 indrical, or bluntly fusiform, or somewhat clavate (fig, 37, a). 

 This form is in all cases due to the fact that the corallites at 

 all points of the corallum spring perpendicularly from a central 

 elongated cone, one end of which is closed, and is consequently 

 usually completely covered with the tubes, while the other end 

 is open. Sometimes both ends of the colony are open. The 

 conical space in the centre of the corallum is, in all my speci- 

 mens, filled with calcite or with the surrounding matrix, or is 

 simply, hollow ; but I entertain no doubt that Mr James is 



