BRYOZOA. 355 



Further, why is it impossible to detect a divisional line between 

 the supposed parasite and the "disfigured"' Feuestella? Upon 

 what ground does Mr. Shrubsole base his assertion that the 

 same parasite is found upon brachiopods and crinoids? In this 

 country it is less erratic, besides being an exceedingly- constant 

 type, Hewitrypa fenestellids ranging from the Upper Silurian 

 to the Carboniferous. Lastly if the cross bars of Unit ry pa. are, 

 as claimed by Mr. Shrubsole, the tabulae of a coral, will he not 

 oblige us by publishing a description of the unique form? But 

 let us pass on to something more interesting than this rather 

 impertinent inquisition. 



In Semicoscinium we have a type that commenced already, in 

 the Niagara group, with such aberrant forms of Fenestella as 

 F. tenuiceps and F. acmea. In all its representatives the keel is 

 very high, while, especially in the more typical forms, the 

 branches are not rigid on the reverse side of the zoarium but 

 appear to inosculate. The zoarium is funnel-shaped, with the 

 upper portion more or less undulated and often decumbent. 

 The poriferous side is the outer one, and the strong keels of the 

 branches give this surface a vertically lined appearance. The 

 base of the funnel, particularly in old examples, is smooth and 

 spreads somewhat over the body to which it is attached. The 

 smoothness is due to a thin membrane which is spread over 

 the keels sometimes a considerable distance up the side of the 

 inverted cone, the space between the keels and under the mem- 

 brane being filled with vesicular tissue. The keels themselves 

 are very thin immediately above the top of the branches, but 

 are much thickened towards the summit, which in some cases 

 is quite flat, but in others varies from obtusely to acutely 

 ridge-shaped. The sides of the flattened keel summit is usually 

 smooth, but in some they are crenulated (S. planodorsatum 

 Ulrich. PL 44, fig. 3). By producing these crenulations into 

 transverse connecting bars, we realize through certain interme- 

 diate species (e. g. Fenestella ? precursor Hall, and F. bigeneris 

 Ulr.) what Hall has described as Unitrypa. The transverse 

 bars that characterize this genus are really vertical plates \\hich 

 occur at very regular intervals (corresponding to the zooecia) 

 and are usually, if not always, suddenly bent backward, so as 

 to give the intervening spaces the appearance of imbricating 



